/X+3y=21

Solution for /X+3y=21 equation:

/X+3X=21

We move all terms to the left:

/X+3X-(21)=0


Domain of the equation: X!=0

X∈R


We add all the numbers together, and all the variables

3X+/X-21=0

We multiply all the terms by the denominator


3X*X-21*X+=0

We add all the numbers together, and all the variables

-21X+3X*X=0

Wy multiply elements

3X^2-21X=0

a = 3; b = -21; c = 0;
Δ = b2-4ac
Δ = -212-4·3·0
Δ = 441
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{441}=21$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-21}{2*3}=\frac{0}{6}
=0
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+21}{2*3}=\frac{42}{6}
=7
$