This is a proof by induction.

Base case: When $n = 1,$ $(x+y)^1 = x + y.$ The sum is $$\begin{align} \sum_{i=0}^1 {1 \choose i}x^iy^{1-i} & = {1 \choose 0}x^{1-0}y^0 + {1 \choose 1}x^{1-1}y^1 \\ & = x + y \end{align}$$ So, the theorem is true for $n = 1.$

When $n = 2,$ $(x+y)^2 = x^2 + 2xy + y^2.$ The sum is $$\begin{align} \sum_{i=0}^2 {2 \choose i}x^iy^{2-i} & = {2 \choose 0}x^{2-0}y^0 + {2 \choose 1}x^{2-1}y^1 + {2 \choose 2}x^{2-2}y^2 \\ & = x^2 + 2xy + y^2 \end{align}$$ So, the theorem is true for $n = 2.$

Inductive step: Suppose the theorem is true for $n = k.$ We will show it is true for $n = k+1.$ $$\begin{align} (x+y)^{k+1} & = (x+y)(x+y)^k \\ & = (x+y)\sum_{i=0}^k {k \choose i}x^{k-i}y^i \end{align}$$ The last equality follows from the inductive hypothesis.

Next, distribute the sum over $x+y.$ $$\begin{align} (x+y)\sum_{i=0}^k {k \choose i}x^{k-i}y^i & = \sum_{i=0}^k {k \choose i}x^{k-i+1}y^i + \sum_{i=0}^k {k \choose i}x^{k-i}y^{i+1} \\ & = \sum_{i=0}^k {k \choose i}x^{k-i+1}y^i + \sum_{i=1}^{k+1} {k \choose i-1}x^{k-i+1}y^i \\ & = x^{k + 1} + \sum_{i=1}^k {k \choose i}x^{k-i+1}y^i + \sum_{i=1}^k {k \choose i-1}x^{k-i+1}y^i + y^{k+1} \\ & = x^{k+1} + \sum_{i=1}^k \left[{k \choose i} + {k \choose i-1}\right]x^{k-i+1}y^i + y^{k+1} \\ \end{align}$$ Next, compute the sum of combinations as follows: $$\begin{align} {k \choose i} + {k \choose i-1} & = \frac{k!}{i!(k-i)!} + \frac{k!}{(i-1)!(k-i+1)!} \\ & = \frac{(k-i+1)k!}{i!(k-i+1)!} + \frac{ik!}{(i!(k-i+1)!} \\ & = \frac{(k+1)k!}{i!(k-i+1)!} \\ & = \frac{(k+1)!}{i!(k+1-i)!} \\ & = {k+1 \choose i} \end{align}$$ Using this identity, we have $$\begin{align} x^{k+1} + \sum_{i=1}^k \left[{k \choose i} + {k \choose i-1}\right]x^{k-i+1}y^i + y^{k+1} & = x^{k+1} + \sum_{i=1}^k {k+1 \choose i}x^{k+1-i}y^i + y^{k+1} \\ & = \sum_{i=0}^{k+1} {k+1 \choose i}x^{k-i+1}y^i \end{align}$$ which completes the proof.

Use the binomial theorem with $3x$ in place of $x$ and $3$ in place of $y.$ $$\begin{align} (2x + 3)^4 & = {4 \choose 0}(2x)^4(3)^0 + {4 \choose 1}(2x)^3(3)^1 + {4 \choose 2}(2x)^2(3)^2 + {4 \choose 3}(2x)^1(3)^3 + {4 \choose 4}(2x)^0(3)^4 \\ & = {4 \choose 0}16x^4 + {4 \choose 1}24x^3 + {4 \choose 2}36x^2 + {4 \choose 3}18x + {4 \choose 4}81 \end{align}$$ One can use Pascal's triangle to find the values of the combinations:
The combinations in row $4$ are $1, 4, 6, 4, 1.$ $$\begin{align} & {4 \choose 0}16x^4 + {4 \choose 1}24x^3 + {4 \choose 2}36x^2 + {4 \choose 3}18x + {4 \choose 4}81 \\ & = (1)16x^4 + (4)24x^3 + (6)36x^2 + (4)18x + (1)81 \\ & = 16x^4 + 96x^3 + 216x^2 + 72x + 81 \end{align}$$