Final answer:
The equation of the line is y = (1/3)x - 21.
Step-by-step explanation:
The slope-intercept equation for a line is in the form y = mx + b, where m is the slope and b is the y-intercept.
In this case, the slope is 1/3.
The point (3, -20) is on the line, so we can substitute these values into the equation to solve for b.
Using the equation y = (1/3)x + b and substituting (3, -20) for (x, y), we get -20 = (1/3)(3) + b.
Solving for b, we get b = -21.
Therefore, the equation of the line in slope-intercept form is y = (1/3)x - 21.