31.5=3.5b^2

Solution for 31.5=3.5b^2 equation:

31.5=3.5b^2

We move all terms to the left:

31.5-(3.5b^2)=0

We get rid of parentheses

-3.5b^2+31.5=0

a = -3.5; b = 0; c = +31.5;
Δ = b2-4ac
Δ = 02-4·(-3.5)·31.5
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$b_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-21}{2*-3.5}=\frac{-21}{-7}
=+3
$
$b_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+21}{2*-3.5}=\frac{21}{-7}
=-3
$