Equation needs URGENT checking

Hello, i was preparing for a test i have tomorrow so was doing review questions in my text book. I came across this one question where i knew what to do but got the wrong answer, must be a mistake in my working out

Question: the distance between (2,-7) and (a,-2) is units. Find the value of a if it is positive.

Edit: New to using Latex - sorry for any inconvenience

AND

The answer i have been obtaining is a=4

The formula to use was d =

in this case d being

the actual answer is a= 6..