We know the equation of a line is y=mx+b. We know the slope (m) is -2 and all we need is the intercept (b).
Since we know one point, we can plug in 6 for x and 3 for y (along with -2 for m) in our equation and then solve for b.
3= -2*6 + b
3= -12 + b
15 = b
Since we now know the intercept we can now write our equation:
y= -2x + 15
