X=36-y^2

Solution for X=36-y^2 equation:

X=36-X^2

We move all terms to the left:

X-(36-X^2)=0

We get rid of parentheses

X^2+X-36=0

a = 1; b = 1; c = -36;
Δ = b2-4ac
Δ = 12-4·1·(-36)
Δ = 145
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-\sqrt{145}}{2*1}=\frac{-1-\sqrt{145}}{2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+\sqrt{145}}{2*1}=\frac{-1+\sqrt{145}}{2}
$