Question

8rx-4x^2=r^2√(27)
how to isolate x or x^2

Answer 1

Assuming r is a constant you need to put it in a form:
\[ax^2+bx+c=0\]
Then use the quadratic formula (i.e., complete the square)

Answer 2

no, there are two variables x and r. how would you isolate x?

Answer 3

you can still use what @malevolence19 says by completing the square...