The best way to solve this problem is to graph all variables, velocity being the dependent variable (y-axis) and time being the independent variable (x-axis) . Since we are given two points for the two variables, we use the point-slope formula to get the slope which happens to be -1. We then use the standard form y = mx + b and substitute m with -1. Next we want to find b, we will again use the point slope formula now knowing m to be -1. we chose point 0 for time(x-axis) and at zero, we obtain a value of your b which happens to be +19. A line graph would better visualize my point. Cheers!