A certain number added to its square is 30

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Let x be the number. We have:
x^2 + x = 30

Subtract 30 from each side:
x^2 + x - 30 = 0

Using our quadratic calculator, we get potential solutions of:
x = 5 or x = -6

Check 5:
5 + 5^2 = 5 + 25 = 30

Check -6
-6 + -6^2 = -6 + 36 = 30