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)=4

u_{s}(t)=\left\{\begin{array}{ll} 0 & \text { for } t<0 \\ 4 & \text { for } t \geq 0 \end{array}\right.