I can't give a very good answer as I don't know what resources are available to you, but this is what I managed to find with a ti-84 calculator.
[tex]y=-31.24+1.34x[/tex]
If you want a better step-by-step, tell me what's in that 'open statistics calculator' option and I'll see what I can figure out.
edit: missed that second part of the question
Using this equation to calculate a test grade with a homework grade of 90:
[tex]y=-31.24+1.34(90)=89[/tex]
