I don't know how to do this can someone explain it

You’re trying to find x which is the side opposite to the angle. You’re given the adjacent angle. To find the variable, you use tangent. So you plug in tan(x)= opposite/adjacent. In this case it would be tan(27)=x/15. Then you multiply both sides by 15 to be 15(tan(27))=x which is 7.6=x