We have f(x) = (3x + 1)*e^-x

We use the product rule to find f'(x)

f'(x) = (3x + 1)'*e^-x + (3x + 1)*(e^-x)'

=> 3*e^-x - (3x +1)e^-x

=> 3*e^-x - f(x)

f''(x) = -3e^-x - f'(x)

=> -3e^-x - 3e^-x + f(x)

Now f(x) + 2f'(x) + f''(x)

=> f(x) + 2(3*e^-x - f(x)) + (-3e^-x - 3e^-x + f(x))

=> f(x) + 6e^-x - 2f(x) - 2*3e^-x + f(x)

cancel the common terms

=> 0

As f(x) + 2f'(x) + f''(x) = 0

=> f(x) = -2f'(x) - f''(x)

**This proves that f(x) = -2f'(x) - f''(x)**