2m + 57 = -8m + 7
add 8m to each side to get the variables on one side
2m+8m +57 = -8m+8m +7
10m +57 = 7
subtract 57 from each side to the constant on the other side
10m +57-57 = 7-57
10m =-50
divide by 10 on each side to get m alone
10m/10 = -50/10
m = -5
