You can start by writing equations with the given information
1) You know that together, Mindy and Troy ate 9, so you can write
M+T=9
2) Then, it says that Mindy ate 3, so M=3
3) Troy ate 1/4 of the total cake, however we don't know what the amount of the total cake is, so we can represent it as x, T=1/4x
4) Plug in these equations into the first equation, 3+1/4x=9
5) Solve for X, Subtract 3 from both sides to get 1/4x=6
6) Divide 1/4 on both sides, x=(6÷1/4)
x=24
Hope this helped :)