(a) Work done = Area under curve and displacement axis
= Area of trapezium
$= \frac{1}{2} \times (sum\ of\ two\ parallel\ lines) \times distance\ between\ them$
$= \frac{1}{2}\ (10 + 4) \times (2.5 - 0.5) = \frac{1}{2}14 \times 2 = 14\ J$
As the area actually is not trapezium so work done will be more than $14\ J$ $i.e.$ approximately $16\ J$