Answer :
for all the equations you substitute the given values of n.you will then subtract the product of 3n from both sides. so you will end up with three values of M for this question.so:
M+3(-2)=7
M-6+6=7+6
M=7+6
M=13
M+3(0)=7
M=7
M+3(1)=7
M+3-3=7-3
M=7-3
M=4
M+3(-2)=7
M-6+6=7+6
M=7+6
M=13
M+3(0)=7
M=7
M+3(1)=7
M+3-3=7-3
M=7-3
M=4