Since 3e^(-mt) 5t cosh(at) = (-3e^(-mt))((s-m)^2)+(5(-s^2-a^2))/((s^2-a^2)^2), to find the Laplace transform of 3e^(-mt) 5t cosh(at), you negate the derivative of (-3e^(-mt))((s-m)^2)+(5(-s^2-a^2))/((s^2-a^2)^2).
Therefore L{ 3e^(-mt) 5t cosh(at)} (s) = -((-3e^(-mt))((s-m)^2)+(5(-s^2-a^2))/((s^2-a^2)^2))