Finding the voltage on a capacitor
You've made an excellent start.

You have calculated the value of C1+4//2+5 = 40nF, and you know C3. This will allow you to calculate the potential at their common node, as whatever current flows through that node from C3 flows into the C1+4//2+5 effective capacitor.

Rinse and repeat for C1+4, and the value of C2.

This will give you the voltage at each end of C2, the difference is what you require.

Be aware that the learning intention of this exercise was not to calculate the voltage on C2 per se. It was a) to get you to recognise a ladder of impedances, b) to do as you did which was to cascade the impedances up from the bottom, until you find the effective impedance at the first node (40nF) c) from which you can get the voltage at that first node (18v) d) from which you can calculate back down to get the voltage a node at a time.

This is a ladder. The simplest non-trivial network of components.