Arithmetic regarding area under curve

Question

I am looking at a textbook example but I am not able to go from where I have set the question mark and to the where the arrow is pointing. Could someone please explain where the x's are coming from and how they have done it?

Answer 1

You are missing some context. It is probably written somewhere that $x(t)=v_0t+\frac a2 t^2$ ($x$ is a function of $t$).