Y=mx+b is a way to find out the y intercept on a graph.
M= slope
B= y intercept
To figure it out, you have to have a slope and 2 coordinates that go together (a coordinate point). Let's say the slope is 4, and the coordinate is (3,5)
Now, plug in all your know information.
5= 4(3) +b
5= 12 + b
Subtract 12 from both sides
-7 = b