Math

Let F = the father's age.
Let D = the daughter's age.

Here's what we know:

F = 4D
F + 6 = 3(D + 6)

Substituting the first of these into the second gives:

4D + 6 = 3(D + 6)
4D + 6 = 3D + 18

Subtracting 3D from both sides gives:

D + 6 = 18

Subtracting 6 from both sides gives:

D = 12

And since the first equation tells us that F = 4D, we know that F = 4(12) = 48

The father is 48 and the daughter is 12.