Hi
Using the standard slope-intercept form for an equation of a line y = mx + b
where m is the slope and b the y-intercept.
y = A+B(x-3)
y = Bx - 3B + A | ⊥ to y = 2x, therefore it's slope will be m = -1/2
B = -1/2
y = (-1/2)x +3/2 + A |using ordered pair pt(5,15) to solve for A
15 = -5/2 + 3/2 + A
16 = A
CHECKING our Answer
y = 16 -(1/2)(x-3)
15 = 16 -(1/2)(5-3)
15 = 16 - 1