Answer: u = 1/24
Explanation:
-7/8u = -21
Multiply both sides by 8u
(8u) -7/8u = -21 (8u)
-7 = -21 (8u)
Divide by -21
-7/-21 = 8u
1/3 = 8u
Divide by 8
(1/3)/8 = u
When dividing fractions, flip and multiply
(1/3) (1/8) = u
u = 1/24
It works if you plug it back in, hope this helps