I wanted to plot $\displaystyle xy(x^2-y^2)=C$ for C=840 but realised I couldn't isolate either x or y

I went to quickmath.com and entered the general equation and got...

Solve Equation :: QuickMath.com - Automatic Math Solutions

This returned 3 equations for calculating y; 2 complex and 1 real.

I know that for $\displaystyle xy(x^2-y^2)=840$ (7, 3), (7, 5) and (8, 7) are co-ordinates on the curve.

I cannot get these results by plugging in C=840, x=7 to the equations returned by quickmath.com.

Am I doing something wrong?

Thx

Ben