We have to find derivative of t
[tex]\\ \sf\longmapsto \dfrac{d}{dx}3t^2-12t+4[/tex]
[tex]\boxed{\sf \dfrac{d(x^n)}{dx}=nx^{n-1}}[/tex]
[tex]\\ \sf\longmapsto \dfrac{d}{dx}3t^2-\dfrac{d}{dx}12t+\dfrac{d}{dx}40[/tex]
[tex]\\ \sf\longmapsto 6t-12[/tex]
Now
[tex]\\ \sf\longmapsto 6(8)-12[/tex]
[tex]\\ \sf\longmapsto 48-12[/tex]
[tex]\\ \sf\longmapsto 36[/tex]