Alright, lets get started.
Suppose the original square window has the side = x feet
The new window is 3 ft wider and 2 ft higher than the square one.
Means length of the new window will be =
Width of the new window will be =
So the area of the new window will be =
So the area of the new window will be =
So the area of the new window will be =
As given in question the area of 30 square feet for the new window, hence
Factoring
x = -8 or x = 3
Side could not be negative hence
side of the original square window will be = 3 feet : Answer
Hope it will help :)