slope and y intercept help please!! Test in the morning!

I need some help ASAP!! I SUCK at math, have a test first thing tomorrow morning... well, I'm reviewing and run into a road bump.

Here's my problem:

A company finds that it can produce 25 heaters for $6500, while producing 45 heaters for $9000.

Express the cost, y, as a linear function of the number of heaters, x.

Ok... So I've got that my points are (25,6500) and (45,9000), and I've found my slope to be 125 using the y2-y1 divided by x2-x1.

So, I've got part of the problem solved.

But I have to write it out as y=mx+b

So I only have y=125x+b

How in the heck do I find the y intercept (b)?

My answer key says it should be 3375 but how the heck did they get that? I feel like I know this but I can't remember for the life of me! Please help if you can!!