7=3x*5x

Solution for 7=3x*5x equation:

7=3x*5x

We move all terms to the left:

7-(3x*5x)=0

We add all the numbers together, and all the variables

-(+3x*5x)+7=0

We get rid of parentheses

-3x*5x+7=0

Wy multiply elements

-15x^2+7=0

a = -15; b = 0; c = +7;
Δ = b2-4ac
Δ = 02-4·(-15)·7
Δ = 420
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{420}=\sqrt{4*105}=\sqrt{4}*\sqrt{105}=2\sqrt{105}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-2\sqrt{105}}{2*-15}=\frac{0-2\sqrt{105}}{-30}
=-\frac{2\sqrt{105}}{-30}
=-\frac{\sqrt{105}}{-15}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+2\sqrt{105}}{2*-15}=\frac{0+2\sqrt{105}}{-30}
=\frac{2\sqrt{105}}{-30}
=\frac{\sqrt{105}}{-15}
$