velocity time graph

I cannot see the rest of question B

Here's the first part

v = u + at

Note that a = change in velocity / change in time

So...... change in velocity  / change in time =  (v  - u ) / t  

So we have that

v =   u + at

v = u +  [ ( v - u) / t ]  * t   

v = u + (v - u)

v = u + v - u

v =  v

Second part

The total displacement, s,  is the green area

Notice that the total  shaded area of the bottom  rectangle is   u* t

And the area of the top rectange  is (v - u)* t

But only half of this rectangle is shaded...so....the shaded area in the top rectangle is  (1/2)(v - u) * t

So.....the displacement is

u * t + (1/2) ( v - u) *t  =

u * t +  (1/2)v*t  - (1/2)u*t =

1/2 v* t   + 1/2 u*t =

(1/2) ( u + v) * t    =   s