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(7x-4)(3x+21)=0
We multiply parentheses ..
(+21x^2+147x-12x-84)=0
We get rid of parentheses
21x^2+147x-12x-84=0
We add all the numbers together, and all the variables
21x^2+135x-84=0
a = 21; b = 135; c = -84;
Δ = b2-4ac
Δ = 1352-4·21·(-84)
Δ = 25281
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{25281}=159$$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(135)-159}{2*21}=\frac{-294}{42} =-7 $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(135)+159}{2*21}=\frac{24}{42} =4/7 $
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