How many mols of O2 are needed to completely react with 7 mols of C2H2?

I have already balanced - 2C2H2+5O2->4CO2+2H2O what do I do next?

2 moles acetylene--------------5 moles oxygen O2

7 moles acetylene--------------x moles oxygen O2

x = 7 x 5/2 = 17,5