Question

9=7d+8(9-2d)

A d=-1/9

B d=-9

C d=7

D=1/7

Answer 1

$9=7d+8(9-2d)\\9=7d+72-16d\\9=-9d+72\\-72+9=-9d\\-63=-9d\\d=7$

Also for the 8(9-2d), simplify by multiplying the 8 into the (9-2d) as we get (8*9)-(8*2d) = 72-16d

Therefore, the answer is d = 7 aka C.