-4c-11=4c+21

move constants to one side and variables to the other.

add -21 to both sides and add +4c to both sides.

-4c+4c-11-21 = 4c+4c+21-21

-11-21 = 8c

combine terms

-32 = 8c

divide both sides by 8

-32/8 = 8c/8

-4 = c

Check my work. I always check the answer, too, by substituting into the original. In this case we have

(-4*-4) - 11 = (4*-4)+21

+16-11 = -16+21

+5 = +5

GOOD. It checks.