Use the distributive property and distribute 8 into c and 9 on the left side of the equation and distribute 6 into 2c and 12, then follow the order of operation to solve.
8(c-9) = 6(2c - 12) - 4c
8c - 72 = 12c - 72 - 4c
8c - 72 = 8c - 72, which is true