Answer:
(0, -13/2) or (0, -6.5)
Explanation:
a point on the y-axis means x=0.
so, the solution is a point (0, y).
and I assume (5, 6) is not R but is B.
for the angle at B being 90 degrees, we are looking at a right-angled triangle, and basic Pythagoras has to be true :
c² = a² + b²
with c being the Hypotenuse (the line opposite of the angle with the 90 degrees).
in our case, this c is the connection from A to C.
and a and b are the connections AB and BC.
so,
AC² = AB² + BC²
the distance between two points is again calculated via Pythagoras, where the differences in x and y directions are the sides, and the distance itself is the Hypotenuse.
so, we get
(0-0)² + (8-y)² = (0-5)² + (8-6)² + (5-0)² + (6-y)²
(8-y)² = -5² + 2² + 5² + (6-y)²
64 - 16y + y² = 25 + 4 + 25 + 36 - 12y + y²
64 - 16y + y² = 90 - 12y + y²
64 - 16y = 90 - 12y
-26 = 4y
y = -26/4 = -13/2 = -6.5