3. Solve the equation $9^{1+2 \sqrt{x}}-28 \cdot 9^{\sqrt{x}}+3=0$.
# Solution:

1) $9^{\sqrt{x}}=\frac{1}{9}$, no solutions;
2) $9^{\sqrt{x}}=3, \sqrt{x}=\frac{1}{2}, x=\frac{1}{4}$.
Answer: $x=\frac{1}{4}$.