y(y-10)=21

Solution for y(y-10)=21 equation:

y(y-10)=21

We move all terms to the left:

y(y-10)-(21)=0

We multiply parentheses

y^2-10y-21=0

a = 1; b = -10; c = -21;
Δ = b2-4ac
Δ = -102-4·1·(-21)
Δ = 184
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{184}=\sqrt{4*46}=\sqrt{4}*\sqrt{46}=2\sqrt{46}$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-10)-2\sqrt{46}}{2*1}=\frac{10-2\sqrt{46}}{2}
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-10)+2\sqrt{46}}{2*1}=\frac{10+2\sqrt{46}}{2}
$