diff eqn.
posted by Farah .
y(t) = 0.2451 cos 4t + 0.9804 sin 4t + e^(3t ) (A cos〖t+B〗 sin〖t)〗
y'(t) = ?

You are not being asked to solve a differential equation. You only have to take the derivative of the y(t) function you are given.
Just add the derivatives of the three separate terms.
The derivative of
0.2451 cos(4t)
is
0.9804 sin(4t)
Your third term is illegible.