After letting Samual's age be S, the correct equations you're looking for are:
M = 4S ------- eq (i)
4S + 5 = 3(S + 5) - 6 ------- eq (ii)
After distributing and simplifying eq (ii), substitute 4S for M in eq (ii) to get:
FYI: Negative ages or ages < 0 DO NOT exist, so ignore any "solution" that represents an age as being < 0.