Since is constant with respect to , move out of the integral.

Let . Find .

Differentiate .

Since is constant with respect to , the derivative of with respect to is .

Differentiate using the Power Rule which states that is where .

Multiply by .

Rewrite the problem using and .

Since is constant with respect to , move out of the integral.

Multiply by .

The integral of with respect to is .

Simplify.

Replace all occurrences of with .

Evaluate integral of 3e^(-x) with respect to x