I do not know how the 1's cancel out to give 1/3^x

Unfortunately your teacher is using x as both a variable and a multiplication sign. This is something that can be avoided by using something like the asterisk symbol to indicate multiplication.

Anyways, notice how the expression shows up twice. Once in the numerator (the entire numerator) and once again in the denominator (nearly the whole thing)

Let's replace that messy expression with the variable y. So we're letting

This means,

At this point you can probably see how to get from here. The y terms cancel out when we divide leaving 1 up top and 3^x down below.

Complete the missing parts of the paragraph proof.

By the definition of  

supplementary

angles, the sum of the measures of angles 1 and 2 is 180 degrees. Likewise, the sum of the measures of angles  

3 and 2

is 180 degrees. By the  

substitution

property, mAngle1 + mAngle2 = mAngle3 + mAngle2. Subtract the measure of angle  

2

from each side. You get mAngle1 = mAngle3, or Angle1 Is-congruent-to Angle3, by the definition of congruence

Step-by-step explanation: