The lengths of the sides of a triangle are \( 10 + x^2 \), \( 10 + x^2 \) and \( 20 - 2x^2 \). If for \( x = k \), the area of the triangle is maximum, then \( 3k^2 \) is equal to:
\( 5 \)
\( 8 \)
\( 10 \)
\( 12 \)
