The mum is 21 years older than the child.
M = C + 21

In 6 years from now the mum will be 5 times as old as the child.
M + 6 = 5 x (C + 6)

Hence,
C + 21 + 6 = (C + 6) x 5
C + 27 = 5C + 30
-3 = 4C
C = -3/4