7. x + 4x = f(t) with no initial conditions (x(0)=0 and x(0) = 0) and where f(t) is a step function of amplitude 4, that is, f(t)= 4u, (t)=\left\{\begin{array}{ll}

0 & \text { for } t<0 \\ 4 & \text { for } t \geq 0 \end{array}\right.