To find acceleration we use the formula a=(Vf-Vi)/T where a is acceleration Vf is the final velocity Vi is the initial velocity and T is time. Since we start at 20 seconds and end at 40 seconds, the time elapsed is 40-20=20 seconds. Our initial velocity is 15,000 m/s and our final velocity is 17,500 m/s. Plugging these values into our equation, we get a=(17,500 m/s-15,000 m/s)/20 s.
Our next step is to solve the subtraction in the parentheses:
a=(2,500 m/s) /20s
Next we divide:
Notice that since the velocities were already expressed in terms of m/s, and we have to divide by the time, which is expressed in seconds, our final answer is expressed in terms of m/s2.