$\bullet$ $\bullet$ The graph in the accompanying figure shows the magni-
tude of the force exerted by a given spring as a function of the
distance $x$ the spring is stretched. How much work is needed to
stretch this spring: (a) a distance of $5.0 \mathrm{cm},$ starting with it
unstretched, and (b) from $x=2.0 \mathrm{cm}$ to $x=7.0 \mathrm{cm} ?$