# Step-by-step Solution

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## Step-by-step explanation

Problem to solve:

$x^2-3x-4=\left(x+1\right)\left(x-4\right)$
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Grouping all terms to the left side of the equation

$x^2-3x-4-\left(x+1\right)\left(x-4\right)=0$
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The product of two binomials of the form $(x+a)(x+b)$ is equal to the product of the first terms of the binomials, plus the algebraic sum of the second terms by the common term of the binomials, plus the product of the second terms of the binomials. In other words: $(x+a)(x+b)=x^2+(a+b)x+ab$

$x^2-3x-4-\left(x^2+\left(1-4\right)x+1-4\right)=0$

$x^2-3x-4=\left(x+1\right)\left(x-4\right)$