If |3x + 1| = 13 and x is positive, what is the value of x?

Split $|3x + 1| = 13$ into two cases. Case 1: $3x + 1 = 13$ gives $3x = 12$ and $x = 4$. Case 2: $3x + 1 = -13$ gives $3x = -14$ and $x = -\frac{14}{3}$. The problem says $x$ is positive, so only $x = 4$ qualifies, choice D. Check: $|3(4) + 1| = 13$. Choice A ($-\frac{14}{3}$) is the valid negative solution, excluded by the condition. Choice B ($\frac{13}{3}$) comes from dividing 13 by 3 and ignoring the $+1$. Choice C ($\frac{14}{3}$) drops the negative sign from the second case, which does not satisfy the equation.