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:

$\displaystyle

\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 $\displaystyle \frac{P}{D(p)} * D'(p)$