Who knows how to do this?

If f(x)=x^3 and g(x)=2x+7 when g(x)=-5

Tim's phone service charges $26.39 plus an additional $0.21 for each text message sent per month. if tim's phone bill was $31.64, which equation could be used to find how many text messages, x, tim sent last month?

Ineed someone to me answer my question i have to have this done and knocked out