(80000-y^2)÷200=(3210-ky)÷5

$$\\\frac{(80000-y^2)}{200}=\frac{(3210-ky)}{5}\\\\
$multiply both sides by 200$\\\\
80000-y^2=40(3210-ky)\\\\
80000-y^2=128400-40ky\\\\
0=y^2-40ky+128400-80000\\\\
y^2-40ky+48400=0\\\\$$

 

Now, if you want to make y the subject you can use the quadratic formula.  :)