3x(7x-15)=25

Solution for 3x(7x-15)=25 equation:

3x(7x-15)=25

We move all terms to the left:

3x(7x-15)-(25)=0

We multiply parentheses

21x^2-45x-25=0

a = 21; b = -45; c = -25;
Δ = b2-4ac
Δ = -452-4·21·(-25)
Δ = 4125
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:

$\sqrt{\Delta}=\sqrt{4125}=\sqrt{25*165}=\sqrt{25}*\sqrt{165}=5\sqrt{165}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-45)-5\sqrt{165}}{2*21}=\frac{45-5\sqrt{165}}{42}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-45)+5\sqrt{165}}{2*21}=\frac{45+5\sqrt{165}}{42}
$