Basic formula for the price elasticity of demand coefficient is on this webside:
http://www.economicsdiscussion.net/price-elasticity-of-demand/price-elasticity-of-demand-with-formula/25223
(s÷t) - r is the correct expression.
Answer: B. x, in the second equation
Step-by-step explanation:
The given system :

To apply the substitution method , first we need to select the best variable and a equation which is easy to simplify.
From the given system equation (2) have all the numbers which are multiples of 3.
Then we divide 3 on both sides of equation (2), we get

Therefore, the best variable to solve for and from what equation is x, in the second equation.
You're basically using the supplement angle to find the missing number.