Solution: Let $ u = \sqrtx $, so $ u \geq 0 $. Substituting, the equation becomes $ u + 3u^2 = 4 $. Rearranging: $ 3u^2 + u - 4 = 0 $. Using the quadratic formula: $ u = \frac-1 \pm \sqrt1 + 486 = \frac-1 \pm 76 $. Valid solution: $ u = 1 $ (since $ u \geq 0 $). Thus, $ \sqrtx = 1 \Rightarrow x = 1 $. \boxed1 - Portal da Acústica