Slope-intercept form:
y = mx + b
where m is the slope, and b is the y-intercept
since f(x) = 3x - 15 is already in the slope-intercept form
then the y-intercept is -15.. take note that when a line intersects the y axis, x=0
therefore you'll have the coordinates of the point intersecting the y-axis as
(0,-15)
