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7x^2+21=84
We move all terms to the left:
7x^2+21-(84)=0
We add all the numbers together, and all the variables
7x^2-63=0
a = 7; b = 0; c = -63;
Δ = b2-4ac
Δ = 02-4·7·(-63)
Δ = 1764
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{1764}=42$$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-42}{2*7}=\frac{-42}{14} =-3 $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+42}{2*7}=\frac{42}{14} =3 $
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