Consider the set \( \{ 1, x, x^2 \} \). An orthonormal basis in \( x \in [-1, 1] \) is formed from these three terms, where the normalization of a function \( f(x) \) is defined via
\[
\int_{-1}^{1} \left[ f(x) \right]^2 \, dx = 1.
\]
If the orthonormal basis set is \( \left( \frac{\sqrt{3}}{2}, \frac{\sqrt{5}}{2} x, \frac{1}{2} \sqrt{\frac{21}{N}} (5x^2 - 3) \right) \), then the value of \( N \) (in integer) is: