So... hmmm if you check the first picture below, for 2)
we could use the proportions of those small, medium and large similar triangles like

now.. for 3) will be the second picture below
(1/0)^2+(22/3)
You can't divide anything by 0, so the first part (1/0)^2 is undefined, which makes the entire expression undefined.
Answer:
The difference quotient is the quotient of the difference of the function values, f(x + h) - f(x), and the difference of the input values, (x + h) - x.
Given the function f(x) = -x + 21.
The different quotient will be : 
We see that given the function f(x) = -x+21 and two input values of x and x + h, the difference quotient is -1.