You're doing it right. First we need an equation that shows how x is related to time.
Solve the differential equation. The question states: "Assuming that x = 0 initially".. so x(0)=0. You can find the integration constant from this initial condition. Now we have the equation. Use it to find the answer.
Remember: In the equation, the unit of t is hours as the question says: "At time t hours". Convert the units if needed.