Find the equation of the line through the points (-2,-9) and (5,12)
y =

y - 12 = 3(x - 5)

Step-by-step explanation:

Going from (-2, -9) to (5, 12), x (the run) increases by 7 and y (the rise) increases by 21.  Thus, the slope of this line is m = rise / run = 21/7 = 3.

Using the point-slope formula y - k = m(x - h), we get:

y - 12 = 3(x - 5)

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