When gases are in STP, it doesn't matter what the compound or chemical the gas is: there are 22.4 liters in a mole. No matter what. (Note that this is for STP and STP only! If you get a question like this and it's not in STP, disregard this rule!)
Knowing this, in order to calculate volume all we have to do is multiply it by 22.4:
4.2 * 22.4 = 94.08
There are 94 liters in C2H6 gas at STP
-T.B.
