Use the chain rule:
[tex]\dfrac{\mathrm dy}{\mathrm dx}=\dfrac{\mathrm dy}{\mathrm du}\cdot\dfrac{\mathrm du}{\mathrm dx}[/tex]
With [tex]y=\cos u[/tex], you have [tex]\dfrac{\mathrm dy}{\mathrm du}=-\sin u[/tex].
With [tex]u=7x^2[/tex], you have [tex]\dfrac{\mathrm du}{\mathrm dx}=14x[/tex].
So
[tex]\dfrac{\mathrm dy}{\mathrm dx}=-14x\sin u=-14x\sin(7x^2)[/tex]
