Answer:
see the attachment
Step-by-step explanation:
It helps to follow along the proof and consider how you get from here to there.
In the 5th line, we have a sum in symbols.
In the 7th line, we have an expression with a number on the right side, and a statement that says we got there by subtraction.
Then we expect line 6 to have a sum with numbers whose difference is 40°. (That eliminates the 3rd and 4th choices.)
The way you get from an expression with symbols to an expression with numbers is to make a <em>substitution</em> of numbers for symbols. Hence the appropriate choice is the one shown below.
