The velocity of a body increases uniformly from $4$ m s$^{-1}$ to $10$ m s$^{-1}$ in $3$ s. Find the displacement during this interval.

Displacement is the area under the velocity-time graph, a trapezium: $\dfrac{4 + 10}{2} \times 3 = 21$ m. Using the final velocity alone gives 30 m; using only the change in velocity gives 18 m.