Simple,
using [tex] \frac{y1-y2}{x1-x2} =m (slope)[/tex]
find your slope...
[tex] \frac{5-15}{1-2} = \frac{-10}{-1}=10 [/tex]
thus, 10=m (slope)
Now, using y=mx+b to find your equation, plug in what you know...
Use any of the 2 coordinates.. (1,5)
5= 10(1)+b
5=10+b
-10 -10
-5=b
Thus, the equation to your line is...
y=10x-5.
