f(x)= e−(5x)⋅cos(3x)

Answer 1

Find the derivative

Answer 2

just plug in the product rule and the chain rule

f = e^(-5x)cos(3x)
f' = -5e^(-5x) cos(3x) + e^(-5x) (-3sin(3x))
= -e^(-5x) (5cos3x + 3sin3x)