If I have Y=-3X+21 do i simplify or keep it just the same???

Answer 1

What is the original problem? What does the problem state to do?

You could write this as,
y = 3(x + 7)

Depending on your problem y = 3x + 21 is acceptable, especially if you are learning slope-intercept form of an equation.

But you wouldn't be wrong to write this as, (if you are learning factoring).
y = 3(x + 7).