Given:
total = 750
ratio = 7 : 8
We need to get the ratio of the number to its total
7 + 8 = 15 ; 7/15 and 8/15 is the ratio of the number to its total.
We multiply these ratio to the total of 750
7/15 * 750 = 350
8/15 * 750 = 400
7:8 is equal to 350:400
The area of square: s · s = s²
The area of rectangle: (s + 6) · s = s(s + 6)
So:
s² + s(s + 6) = 56
s² + s(s + 6) - 56 = 0
Answer D.