Question 1096315
As an equation, this is:

252 = (21 - 5x) * 7

Solve for x:

252 / 7 = (21 - 5x) * 7 / 7
252 / 7 = 21 - 5x
252 / 7 - 21 = 21 - 5x - 21
252 / 7 - 21 = 5x
x = (252 / 7 - 21) / 5

Simplify:

x = (252 / 7 - 21) / 5
x = (36 - 21) / 5
x = 15 / 5
x = 3

Convert to sq ft:

x = 3 sq ft.