The equation for a straight line is :
y = m x + b m is the slope of the line
b is the y intercept (the point on the y-axis where straight line crosses it)
For example, what is y- intercept for a straight line containing point ( 4, 6 ) with a slope of 2?
substituting into the straight line equation above
6 = 2*4 + b
6= 8 + b
-2 = b the line crosses the y-axis at the -2 mark
Hope this helps!