I am struggling with a question regard parametric curves and finding tangents to them but something is going wrong somewhere in the process and I cannot figure out why. 

The question asks: consider the parametric curve $x=3\cos(2t)$ and $y=t^{7/2}$. Find the equation of the tangent at $t=\pi/4$. 

So what I've tried is using the formula $y-y\_1=m(x-x\_1)$. So I've solved $x$ and $y$ for $t=\pi/4$. Used these as my x and y coordinates. Next I've rearranged so that $t=3\arccos(x)/2$ and then substituted that into the equation $y=t^{7/2}$ 

I believe this is where my process becomes dodgy. I've taken the derivative of $x$ and $y$ and then divided $y'/x'$ and used that as my slope. Then I've rearranged the point slope formula and found my equation of the tangent line and provided it in the form of $y=mx + c$ as required. 

I am honest unsure oh what I am missing here because no matter what I try I cannot seem to nail this question despite it being quite simple and straightforward. I think my process is good I am not sure if it is just a simple calculation error along the way. 

Any guidance would be greatly appreciated. 

Thank you.