84=b^2+3b

Solution for 84=b^2+3b equation:

84=b^2+3b

We move all terms to the left:

84-(b^2+3b)=0

We get rid of parentheses

-b^2-3b+84=0

We add all the numbers together, and all the variables

-1b^2-3b+84=0

a = -1; b = -3; c = +84;
Δ = b2-4ac
Δ = -32-4·(-1)·84
Δ = 345
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-3)-\sqrt{345}}{2*-1}=\frac{3-\sqrt{345}}{-2}
$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-3)+\sqrt{345}}{2*-1}=\frac{3+\sqrt{345}}{-2}
$