You are expected to know that the Heaviside Step Function U(t - a) = 1 if t > a and zero if t < a. That is why I asked in my first post!
If you don't know the definition, then at this level you are expected to find a reference that explains it. I would be very very surprised if your textbook or class notes did not define it.
And if you know that U(t) = 1 for t > 0 and 0 for t < 0 then you are expected to understand the effect of a transformation such as U(t) --> U(t - a).