How to calculate the capacity in a system of capacitors
You know that the capacitance of two capacitors C1 and C2 in parallel is C1+C2. 

You know that the capacitance of two capacitors C1 and C2 in series is 
\$ 1 \over {  {1 \over  C1} + {1 \over C2} }\$

You should be able to find the capacitance if the 7.22uF capacitor was not there, using the first equation. 

You should be able to work out the capacitance in series with the unknown capacitance C using a combination of the two equations. 

Then just re-arrange the second equation to find the value of the unknown capacitor.