Draw a large square that has sides a+b. Divide the square into smaller regions as shown below.
The top left region has area a^2 because we multiply the sides 'a' and 'a'
The bottom right region is the same idea, so that's how we get b^2
Leela is partially correct because a^2+b^2 is part of the answer. But there's two missing pieces. Each of those are area of "ab". They combine to 2ab.
Therefore, (a+b)^2 = a^2+2ab+b^2