We use the point-slope form (or, more properly, the two-point form of the equation for a line.
Slope is change in Y divided by change in X. That would be 5-2 divided by 0-3 or 3 over -3 or -1.
Next we identify the y-intercept which is the y value that corresponds to x=0. Lucky for us, we're given that as one of the end points, (0,5), so the y-intercept is 5.
We can put m, the slope, and b, the y-intercept, in the equation y=mx+b to get y=(-1)x + 5.