Could someone please help me with this problem. I have worked most of it out but now I'm not sure what to do.

Multiply ans simplify.
(x-9/4)(x+4/9)
Here is what I have so far:
x(x+4/9)-9/4(x+4/9)=
x^2+4/9x-9/4x-9/4-4/9=
x^2+4/9x-9/4x-1=
This is where I'm not sure what to do.
Could someone please help?
Thanks.

Find common denominator to combine terms.

x^2 + 16/36x - 81/36x - 1.

Does that help?