You can put this solution on YOUR website! Same problem, different numbers.
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log(x) + log(x+21) = 2
log(x*(x+21)) = 2 = log(100)
x^2 + 21x - 100 = 0
(x+25)*(x-4) = 0
x = -25 *** not allowed --> log of negative number
x = 4
