how can you take a line plotted on a graph and make it an equation?

Yes, you just need to know 2 points that are on the line.

If you have the y intercept and one other point then you can find the gradient and use the formula

y=mx+b

where m is the gradient and b is th y intercept

You can either interpolate or extrapolate. Interpolate = find the target point on the line and draw vertical and horizontal lines to see what the X and Y values are. Extrapolate = extend line past it's known area and do the same.