Let M represent Marina’s age. 4 years ago she was M-4 years old.
Let her daughter’s age now be D. 4 years ago she was D-4 years old.
4 years ago, M-4=3(D-4). M-4=3D-12, M=3D-8.
In 6 years’ time their ages will be M+6 and D+6. M+6=2(D+6). M+6=2D+12, M=2D+6.
So we can equate the two expressions for M: 3D-8=2D+6, D=14.
Therefore M=28+6 or 42-8=34. So Marina is 34 years old.