Evaluate
\frac{21}{\left(1-x\right)\left(2x+5\right)}
Factor
\frac{21}{\left(1-x\right)\left(2x+5\right)}
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\frac{3\left(2x+5\right)}{\left(-x+1\right)\left(2x+5\right)}+\frac{6\left(-x+1\right)}{\left(-x+1\right)\left(2x+5\right)}
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of 1-x and 2x+5 is \left(-x+1\right)\left(2x+5\right). Multiply \frac{3}{1-x} times \frac{2x+5}{2x+5}. Multiply \frac{6}{2x+5} times \frac{-x+1}{-x+1}.
\frac{3\left(2x+5\right)+6\left(-x+1\right)}{\left(-x+1\right)\left(2x+5\right)}
Since \frac{3\left(2x+5\right)}{\left(-x+1\right)\left(2x+5\right)} and \frac{6\left(-x+1\right)}{\left(-x+1\right)\left(2x+5\right)} have the same denominator, add them by adding their numerators.
\frac{6x+15-6x+6}{\left(-x+1\right)\left(2x+5\right)}
Do the multiplications in 3\left(2x+5\right)+6\left(-x+1\right).
\frac{21}{\left(-x+1\right)\left(2x+5\right)}
Combine like terms in 6x+15-6x+6.
\frac{21}{-2x^{2}-3x+5}
Expand \left(-x+1\right)\left(2x+5\right).
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