(2x + 16)(2x + 12) = 396
4x2 + 24x + 32x + 192 = 396
4x2 + 56x - 204 = 0
x2 + 14x - 51 = 0
(x + 17)(x - 3) = 0
x + 17 = 0 v x - 3 = 0
x = -17 or x = 3
you can exclude negative value because x cannot be negative :)
Checking:
(16+2*3) * (12 + 2*3) is equal ? 396?
True
So the walkway size is:
22 x 18