M: Michael's age now
R: Richard's age now
Michael is 6 years older than Richard:
M = R + 6
11 years ago, Michael was 4 times as old as Richard was then:
M - 11 = 4 (R - 11)
Replacing M, from the first equation into the second equation:
(R + 6) - 11 = 4 (R - 11)
R + 6 - 11 = 4R - 44
R - 5 = 4R - 44
R - 4R = -44 + 5
-3R = -39
R = -39 / -3
R = 13
Replacing R, in the first first equation:
M = R + 6
M = 13 + 6
M = 19
Michael is 19 and Richard is 13 now.