The equation for slope-intercept form is y=mx+b. To begin, let's figure out the slope.
The slope equation is

0 - 6 / -6 - 3 = -6/-9 = 2/3
The next step is plugging it in to find b.
6 = 2/3(3) + b
Simplify both sides.
6 = 2 + b
Subtract 2 from both sides.
4 = b
Now that we have all the variables, plug in the slope (m) and b, and that should be your equation.
y = 2/3x + 4