To solve the equation \( 3x^2 = 21 \), we can first simplify it.
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Divide both sides by 3:
\[ x^2 = 7 \]
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Now, take the square root of both sides:
\[ x = \sqrt{7} \quad \text{or} \quad x = -\sqrt{7} \]
This means there are two solutions:
- \( x = \sqrt{7} \)
- \( x = -\sqrt{7} \)
Therefore, the number of solutions in the solution set is two.