Answer
$=\frac{5}{s^3}$
Work Step by Step
Given: $f(t)=5e^{-3t}$
Obtain: $F(s)=\int^{\infty}_0 e^{-st}f(t)dt\\
=\int^{\infty}_0 e^{-st} (5e^{-3t})dt\\
=5\int^{\infty}_0 e^{-(s+3)t}dt\\
=5\lim e^{-(s+3)t}+\frac{5}{s+3}e^{-(s+3)}\\
=\frac{5}{s^3}$