To solve the equation \(3x^2 = 21\), we first divide both sides by 3:
\[ x^2 = \frac{21}{3} = 7 \]
Next, we take the square root of both sides:
\[ x = \pm \sqrt{7} \]
This gives us two solutions:
- \(x = \sqrt{7}\)
- \(x = -\sqrt{7}\)
Therefore, there are two solutions in the solution set for the equation \(3x^2 = 21\).