By the chain rule, setting [tex]u=3x^2[/tex], we have
[tex]\dfrac{\mathrm dy}{\mathrm dx}=\dfrac{\mathrm dy}{\mathrm du}\cdot\dfrac{\mathrm du}{\mathrm dx}[/tex]
Since
[tex]\dfrac{\mathrm d(\sec u)}{\mathrm du}=\sec u\tan u[/tex]
[tex]\dfrac{\mathrm d(3x^2)}{\mathrm dx}=6x[/tex]
we end up with
[tex]\dfrac{\mathrm dy}{\mathrm dx}=6x\sec3x^2\tan3x^2[/tex]
