I could really use some help with this function. It's probly really easy but I don't know what I'm doing wrong.

If f(x)=xsquared+3x, find f(2-a).

I did it like this.

f(x)=xsquared+3x

f(2-a)=(2-a)squared+3(2-a)

f(2-a)=4-asquared+6-3a

f(2-a)=-asquared-3a+10

But my teacher told us that it was.

-asquared-7a+10

If you could help me I would really appreciate it. Thanks!