Elasticity=(dD/D)/(dp/p)=(dD/dp)/(D/p).
dD/dp=-4p; D/p=350/p-2p, so elasticity=(-4p)/(350/p-2p)=(-4p2)/(350-2p2).
When p=10, price elasticity=(-400)/(350-200)=-400/150=-8/3.
(You can also work it out by finding dD/dp=-40, and D/p=15, so (dD/dp)/(D/p)=-40/15=-8/3.)