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SAT - Math - Algebra

If x is positive and f(x) = x(x+1) and f(k+5) = 72. What is k?

If x is positive and f(x) = x(x 1) and f(k 5) = 72. What is k? 

f(x) = x (x + 1)

f(k + 5) = (k + 5)*(k +6) = k2 + 11k + 30 = 72

k2 + 11k - 42 = 0

k2 + 14k - 3k - 42 = 0

(k+14) (k-3) = 0

k = 14 or 3