Solve the following initial value problems using Laplace transforms:

a$.\frac{d^{2}y}{d{t}^2}-7\frac{dy}{dt}+10y=0$;$y(0)=0,\frac{dy}{dt}(0)=-3$

b$.\frac{d^{2}y}{d{t}^2}+6\frac{dy}{dt}+8y=3u(t)$;$y(0)=0,\frac{dy}{dt}(0)=0$; where $u(t)=8t$