Caroline,

It depends what information you are starting with. "m" is the slope of the line. You may find this in various ways:

-given

-from two points:
m = (y₁-y₂)/(x₁-x₂)

-from an equation ax + by = c or ax + by + c = 0:
m = -a/b

Once you have m, b is the y-value when x=0. Find it:

-from a known point on the line , by plugging in the point's values of x and y, and the m value you just found
-from an equation ax + by = c, by setting x=0, which makes that term go away, and solving by = c to find y.

Good Hunting!
-RD