5v2-21=10v

Solution for 5v2-21=10v equation:

5v^2-21=10v

We move all terms to the left:

5v^2-21-(10v)=0

a = 5; b = -10; c = -21;
Δ = b2-4ac
Δ = -102-4·5·(-21)
Δ = 520
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$v_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$v_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{520}=\sqrt{4*130}=\sqrt{4}*\sqrt{130}=2\sqrt{130}$
$v_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-10)-2\sqrt{130}}{2*5}=\frac{10-2\sqrt{130}}{10}
$
$v_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-10)+2\sqrt{130}}{2*5}=\frac{10+2\sqrt{130}}{10}
$