Question 1179456
{{{x^2 + 18x + 81 + y^2 - 8y + 16 = 81}}}


You could start, this way:
x^2+18x+81, etc. but in your equation example, completing the square is not needed.  LOOK -
{{{(x^2+18x+81)+(y^2-8y+16)=81}}}------you see, two square trinomials.  FACTORIZE EACH!