Let \( a>0 \). If the function \( f(x)=6x^3-45ax^2+108a^2x+1 \) attains its local maximum and minimum values at the points \( x_1 \) and \( x_2 \) respectively such that \( x_1x_2=54 \), then \( a+x_1+x_2 \) is equal to:
\( 13 \)
\( 18 \)
\( 15 \)
\( 24 \)