M = 4*B
M + 27 = B
Substitute
M = 4*(M+27)
M = 4M + 108
subtract 4M from both sides
-3M = 108
Divide both sides by -3
M = -36
Now we substitute
-36 + 27 =B
-9 = B
Since people can't be negatively aged, we change the sign to positive
So Michael is 36 and Brandon is 9.
Hope this helps :)