90=z2+3z

Solution for 90=z2+3z equation:

90=z2+3z

We move all terms to the left:

90-(z2+3z)=0

We add all the numbers together, and all the variables

-(+z^2+3z)+90=0

We get rid of parentheses

-z^2-3z+90=0

We add all the numbers together, and all the variables

-1z^2-3z+90=0

a = -1; b = -3; c = +90;
Δ = b2-4ac
Δ = -32-4·(-1)·90
Δ = 369
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{369}=\sqrt{9*41}=\sqrt{9}*\sqrt{41}=3\sqrt{41}$
$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-3)-3\sqrt{41}}{2*-1}=\frac{3-3\sqrt{41}}{-2}
$
$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-3)+3\sqrt{41}}{2*-1}=\frac{3+3\sqrt{41}}{-2}
$