Answer:
Logically when looking at it you have to assume x is starting at 0
and a is either a positive or negative number.
Then as x approaches a and the gap between the two becomes ever so tiny, calculate what the logical outcome would be .
for example the first question x-6/x-a
x will approach the value of a and then the result in the denominator will approach 0 or become smaller and smaller as x becomes closer to a. when something is divided by a very small number it will become larger, the smaller the number the larger it becomes. Since the top value will stay 6 as x becomes tiny, the value will move to infinity.
you can test the logic on your calculator, make a 0 and x -0.0000000000000000000000000000000000000001 for the first question to show it approach from the negative direction. this means its aprox 6 over a very small negative number which is -infinity.
