When I tried to graph a graph like this: y=-2x+2 or graphs like that: y=-2x^2+2x+2
But graphs with negative sign in front of the first coefficient would lead to an error.
Are there any ways that I could solve this problem?

I just press the minus sign for the negative sign, but I don't know whether it is right or not.

You're not supposed to use the minus sign for a negative. There's a negative sign on the calculator that you use. It's somewhere near the bottom of the number pad.

Oh now I realized it, I feel stupid that I don't even know that there is a (-) key next to the enter key. THX!

A mod can close this now. :)