Advanced Math practice question

Answer: -5

Substitute x = 3: 9 + 3k − 15 = 0, so k = 2. Then x² + 2x − 15 = (x + 5)(x − 3) = 0, and the other solution is x = −5. (Shortcut: the product of the roots is −15, so the other root is −15 ÷ 3 = −5.)

Common wrong answers

If you answered 2

2 is the value of k, not the other solution.

If you answered 5

5 has the wrong sign: the product of the roots must be −15, so the other root is negative.

If you answered -3

−3 would make the product of the roots −9, not −15.

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