Velocity vs Time

Hello everyone,

I'm having trouble understanding the process behind the following question:

A stationary car is stopped at a traffic light. When the light turns green at t = 0, a truck with constant speed passes the car. The car uniformly accelerates to its top speed in 20 s then moves at a constant speed. The truck is represented by the dotted line and the car is represented by the solid line.

The question itself is clear, but what I don't understand is the following step to the solution:

(35m / s​)(t − 20 s) + 1 / 2 (35m / s​)(20 s)

It seems that if I take the - 20 out of the equation, nothing changes. Could someone explain to me what happens to the - 20 in the step after this one? Thank you so much.