A
We can't really do this without seeing 28, but I can give you an educated guess. The best way for me to proceed is to solve c.
The conjecture is that x = a + b. You should always find that to be true. C is the clincher.
Here's how you do that.
x + c = 180o That's true because all straight lines have 180o. If two angles make up the straight line that means that they are always equal to 180o So x + c = 180o
Now we move to the next step. All triangles also have 180o. That means that a + b + c = 180o
So we have two conditions that equal 180o. Equalities can be equated to one another.
a + b + c = x + c Subtract c from both sides.
a + b = x.
Study what has happened. Put in mathese, the two remote interior angles equal the exterior angle, which is what you are trying to prove.
Summary
a cannot be solved without 28
b you should say that the two remote angles (a and b) will always total x
c The proof is provided for you.