Given,
a-b = 8 --------------- (1)
a+b = -7 --------------(2)
On squaring (1) we get:
(a-b)^2 = 64 ------------------------(3)
On squaring (2) we get:
(a+b)^2 =49 ------------------(4)
On subtracting (3) from (4) we get:
(a+b)^2 - (a-b)^2 = 49-64
or 4ab = -15