A=3x(7x+5)

Solution for A=3x(7x+5) equation:

=3A(7A+5)

We move all terms to the left:

-(3A(7A+5))=0


We calculate terms in parentheses: -(3A(7A+5)), so:

3A(7A+5)

We multiply parentheses

21A^2+15A

Back to the equation:

-(21A^2+15A)


We get rid of parentheses

-21A^2-15A=0

a = -21; b = -15; c = 0;
Δ = b2-4ac
Δ = -152-4·(-21)·0
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$A_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-15}{2*-21}=\frac{0}{-42}
=0
$
$A_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+15}{2*-21}=\frac{30}{-42}
=-5/7
$