[tex]x^2=4(y+3)[/tex]
Differentiate both sides with respect to time [tex]t[/tex]:
[tex]2x\dfrac{\mathrm dx}{\mathrm dt}=4\dfrac{\mathrm dy}{\mathrm dt}[/tex]
We're given that [tex]\frac{\mathrm dy}{\mathrm dt}=6[/tex] at the moment [tex](x,y)=(4,1)[/tex], so that
[tex]2\cdot4\dfrac{\mathrm dx}{\mathrm dt}=4\cdot6\implies\dfrac{\mathrm dx}{\mathrm dt}=3[/tex]
units per second (D).