I'm trying to remember algrebra to help my 4th grade son.

When you have equations like this and you're given one of the variables, all you do is plug that variable value into the equation.

Here we have x=3. That means that in our equation y=3-4x, the x can be replaced with the 3 as follows:

y=3-4(3). Multiply the 4 and 3.

y=3-12. Subtract 12 form 3.

y=-9. Answer.