Simplify completely quantity 12 x plus 36 over quantity x squared minus 4 x minus 21 and find the restrictions on the variable..

To simplify the expression stated, it is better to write it in the numerical form. The statement is:

quantity 12 x plus 36 over quantity x squared minus 4 x minus 21

The numerical form is:

12x+36 / (x²-4x-21)

Simplifying,
12x+36 / (x²-4x-21)
12 (x+3) / (x+3)(x-7)

We can cancel the x+3 term, leaving only:
12/(x-7)

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