Please try to follow me, as I demonstrate a method of solution.
This could get complicated. I'll try to go slowly, and you must
stop me if I do anything that's not clear.
Ready ?
You said that -5c/8 = 20
Multiply each side by 8: -5c = 160
Divide each side by 5 : - c = 32
Multiply each side by -1 : c = -32