78=r2

Solution for 78=r2 equation:

78=r2

We move all terms to the left:

78-(r2)=0

We add all the numbers together, and all the variables

-1r^2+78=0

a = -1; b = 0; c = +78;
Δ = b2-4ac
Δ = 02-4·(-1)·78
Δ = 312
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{312}=\sqrt{4*78}=\sqrt{4}*\sqrt{78}=2\sqrt{78}$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-2\sqrt{78}}{2*-1}=\frac{0-2\sqrt{78}}{-2}
=-\frac{2\sqrt{78}}{-2}
=-\frac{\sqrt{78}}{-1}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+2\sqrt{78}}{2*-1}=\frac{0+2\sqrt{78}}{-2}
=\frac{2\sqrt{78}}{-2}
=\frac{\sqrt{78}}{-1}
$