Question

What is the equation of the line that passes through the points (15,9) and (-2,9)

Answer 1

if you insert both points into the standard form y=mx+b you receive
I:9=15m+b
II:9=-2m+b
subtraction of II from I to eliminate b:
0=17m
m=0 -> line is horizontal as one can also notice by the identical y coordinates :)
insert this in I again:
9=15m+b
9=15*0+b
9=b

-> y=9