http://img42.imageshack.us/img42/171/helphn.png

I seriously in need of help. I found F1 already but i do know how to find X. I know you have to use the Line formula ( Y= Mx + B) but i cant seem to be able to get the answer.

I found the slope for F1 and P, which is .15/.5 = 3/10. Afterward, I plugged into the line formula(y=Mx+b) which is Y= 3/10x+B. I get that you have to plug (-4,0) into B but i do not know how to do it. Any help/ suggestion? This question will be on the future FINAL. Thanks