(-5i)(-3i)=15

Solution for (-5i)(-3i)=15 equation:

(-5i)(-3i)=15

We move all terms to the left:

(-5i)(-3i)-(15)=0

We multiply parentheses ..

(+15i^2)-15=0

We get rid of parentheses

15i^2-15=0

a = 15; b = 0; c = -15;
Δ = b2-4ac
Δ = 02-4·15·(-15)
Δ = 900
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$i_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$i_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{900}=30$
$i_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-30}{2*15}=\frac{-30}{30}
=-1
$
$i_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+30}{2*15}=\frac{30}{30}
=1
$