Respuesta :

4m -21 = 3(5m - 1) 
4m - 21 = 15m - 3
15m - 4m = -21 + 3
11m = -18
m = -18/11

To check: 
LHS → 4 (-18/11)  -21 = - 303/11
RHS → 3( 5(-18/11) - 1) = -303/11
LHS = RHS (The answer is right)