If 3 m equals 12 what is the value of 6(m-1)

Find m in the first equation:
3m = 12
divide both sides by 3:
m = 12/3
m = 4

now yo know m so replace m with 4 in the second equation:

6(m-1) = 6(4-1)
do the subtraction inside the parenthesis first
6(4-1) = 6(3)

now multiply 6 and 3:  6 x 3 = 18