for which value of X will the fraction 7/2x-6 be undefined ?

and check this

IF three times a number decreased by 5, the result is 7. find the number.

3x-5=7
and x=4
is this right?

7/(2x-6) is undefined when the denominator is zero. That would be when x = 3

Your x=4 answer to the second question is correct.