x1+3x2=15

Solution for x1+3x2=15 equation:

x1+3x^2=15

We move all terms to the left:

x1+3x^2-(15)=0

We add all the numbers together, and all the variables

3x^2+x-15=0

a = 3; b = 1; c = -15;
Δ = b2-4ac
Δ = 12-4·3·(-15)
Δ = 181
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-\sqrt{181}}{2*3}=\frac{-1-\sqrt{181}}{6}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+\sqrt{181}}{2*3}=\frac{-1+\sqrt{181}}{6}
$