Question on Elasticity of Demand.

How do we find the value of q, where demand is elastic?

The demand equation is:

p = 21 - 2√q

for 10 >= q =< 90

BOTH SIGNS ARE LESS THAN OR EQUAL AND GREATER THAN OR EQUAL.

The first part of the question is to find the elasticity of demand, which I did get to be:

n = [-2√q (21-2√q)]/(q)

The second part of the question is trying to find the value of q between 10 and 90, for which demand is elastic.

And the third part is computing the total revenue r when the demand has unit elasticity, and 10 >= q =< 90

Thank you for anyone who helps.