The parabola y=x^2 is reflected across the x-axis and then scaled vertically by a factor of 1/6

y = (-1/6)x^2

Step-by-step explanation:

Original function and graph:  y = x^2

Reflection across the x-axis is y = -x^2

Vertical scaling by a factor of 1/6:    y = (-1/6)x^2

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4

6 equilateral triangles