Math Help - Elasticity

1. Elasticity

True or false?
The demand is of unit elasticity at the indicated price p.
D(p) = −1.5p + 7; p = 3

I marked true and it was incorrect. Where did I go wrong?
Here is what I did:
$
\frac{3}{-1.5(3)+7} * -1.5 = -1.8
$

I believe thats the correct setup for elasticity. The absolute value is 1.8 which is > 1. So shouldn't it be elastic?

BTW: The formula that was given for elasticity is $\frac{P}{D(p)} * D'(p)$

2. If the formula

was as you stated then should you have gone $\frac{3}{D(3)}\cdot{D'(3)}=\frac{3}{-1.5\cdot3+7}\cdot{-1.5}=-1.8$ as you indicated?...and since its absolute value is greater than one it is elastic...hmm are you sure you are not omitting any other details?