Let ‘\( a \)’ be a real number such that the function \( f(x)=ax^{2}+6x-15,\ x\in \mathbb{R} \) is increasing in \( \left(-\infty,\dfrac{3}{4}\right) \) and decreasing in \( \left(\dfrac{3}{4},\infty\right) \). Then the function \( g(x)=ax^{2}-6x+15,\ x\in \mathbb{R} \) has a:
