Need help on number 16

A rectangle

Step-by-step explanation:

A rectangle is one kind of quadrilateral with four right angles. The definition of a rectangle is a shape with four sides and four right angles. This means each angle in a rectangle measures 90 degrees. Another property of a rectangle is that opposite sides are parallel to each other and equal in length.

  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 substitution of numbers for symbols. Hence the appropriate choice is the one shown below.