The slope of the given line is the coefficient of x, 3/5. The slope of the perpendicular line will be the negative reciprocal of that: -1/(3/5) = -5/3.
The point-slope form of the equation for a line of slope m through point (h, k) can be written ...
... y = m(x -h) +k
For your point and the slope found above, this becomes
... y = (-5/3)(x -15) -5
When x=0, this is
... y = (-5/3)(-15) -5 = 20
The y-intercept is 20.
