(4 points) Mary's demand curve for food is Q = 10 2P. Her price elasticity of demand for food at price P*equals 2/3 . How much is P*?

I found the question for this

Mary's demand function is QB = 10 2P. His price elasticity of demand at P* is

=( dQ_B/dP)(P * Q_B*)=2 (P *)(10 2P * )

=(-2/3)

3P* = 10 2P *

P* = 2

But how ( dQ_B/dP)(P * Q_B*) became equal to 2 (P *)(10 2P * ) ???

Can you explain step by step ??