Write as a mathematical expression and solve. The sum of a number and 2 times the number is 21. Find the number. i really need help please help

3.69 (rounded)

Step-by-step explanation:

Substitute x for "a number".

New equation: x(x + 2) = 21

Distribute and combine to form a single equation: x^2 + 2x - 21 = 0

Use quadratic formula, factoring, or graphing to get all possible answers.

Quadratic formula: , where A = x^2 coefficient, B = x coefficient, and C = constant.

-5.69, 3.69

Eliminate an incorrect answer that does not line up with the above equation.

3.69

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the answer to the expression is 83