Hi I need help on the following question:

The solution of a certain differential equation is of the formy(t)=acos(3t) + bsin(3t) , wherea andb are constants.

The solution has initial conditions y(0) = 1 and y'(0) = 4

Find the solution by using the initial conditions to get linear equations foraandb

so what did is set y(0) = 1 in the equation and I got a = 1.

I'm having a hard time with getting y'(0)=4