# Laplace Transform Help

## Homework Statement

Find the laplace transform of:

v(t) = sin(7t)u(t-3)

## Homework Equations

u(t) is step response

## The Attempt at a Solution

I know that taken alone the laplace of:

sin(7t) is 7/(s^2+49)

u(t-3) is (1/s)*(e^-3s)

I don't understand how I would figure out the answer to this problem if I have u(t-3). For some reason, seeing the u(t-3) rather than u(t) is really making things hard to understand since I don't know how u(t-3) would be integrated.

I considered setting t-3 =x which would yield:

v(t) =sin(7x+21)u(x)

v(t) = sin(7x)cos(21) + cos(7x)sin(21)

Laplace = [7*cos(21) + s*sin(21)]/[s^2 + 49]

but I don't know if I'm that's the right answer.

Last edited: