$\displaystyle \longrightarrow$ Use rolle's theorem to show that $\displaystyle f(x)=3x-cosx-1$ has only one real root.

So, I know you use IVT to say it has at least one root and then contradict it with Rolle's, but in order to use the IVT you have to pick two points- one negative and one positive so there is point where it was zero.Is there any method for this or do you pick numbers and check to see if it works?