This could be wrong because I'm going into the fifth grade but wouldn't it just be (7,-3)?
Sum 81 to both sides to get

Divide both sides by 4 to get

Consider the square root of both sides. Note that this requires the
sign, since
implies
, but also
. So, the solutions are

6/9 just multiply both parts of the fraction by 3
I think it is a Convience Sampling.