If an object increases velocity at a uniform rate from 7 m/s to 21 m/s in 10 seconds, what is its acceleration and how far does it travel?

Sketch a velocity vs. clock time graph for an object whose initial velocity is 7 m/s and whose velocity 10 seconds later is 21 m/s. Explain what the slope of the graph means and why, and also what the area means and why.