If you know the slope and one point (x,y) of a linear equation to find the y-intercept you use the next equation (general equation for a linea):

b is the y-intercept
Then, you use the value of the slope (m) and the value of the point (x,y) and substitute those values in the equation above to find b.
Example:
If the slope is 4
Point (4, 5)

y intercept for the example is -11