It sounds like you are saying the diagonal line cuts right through the x-y axis at the origin (or zero point where the x and y axes meet). If so then my answer would be this. There are certain graphs that are basice or common and therefore you can know
their equation without knowing...

Let's look at this in two parts: Numerator and Denominator
Demoninator first b/c its easy
0*z=0
0-2=0
Therefore y0=1 because any number taken to the power 0 is equal to 1
So your denominator is 1
Numerator:
Let's start inside the parenthese to see what we can simplify...

Lala, what your are talking about here is the concept of "Combining Like Terms" and it is a key piece of the puzzle when solving algebric equations.
Combining Like Terms is a process used to simplify an expression or an equation using addition and subtraction of the coefficients...