how to solve equation for which the $360 plus $12 per hour and $279 plus $15 per hour would be the same cost

Can someone me with this problem . show your work .

If f(x)=x+7 and g(x)=1/x what is (fog)(x)

The volume of a spherical hot air balloon v(r) = 4 3 ? r3 changes as its radius changes. the radius is a function of time given by r(t) = 3t. find the instantaneous rate of change of the volume with respect to t at t = 2.