Make sure that whatever you do to one side, you must also do to the other. For each of the given values of n, you must find the corresponding value of M.
M + 3n = 7
For n = -2:
M + 3(-2) = 7
M + (-6) = 7
M - 6 = 7
M - 6 + 6 = 7 + 6 | adding 6 to both sides
M = 13
For n = 0:
M + 3(0) = 7
M + 0 = 7
M = 7
For n = 1:
M + 3(1) = 7
M + 3 = 7
M + 3 - 3 = 7 - 3 | subtracting 3 from both sides
M = 4
Hope this helps! :)