Was this the confusion?
Do you mean from:
Because if that's the last mark then 6cos3x+6 can be re-written as 6+6k (let's say cos3x is k).
so 6+6k/(1+k)^2 which is equal to:
take away a (1+k) from top and bottom and you are left with 6/1+k which is:
Somehow, if you made it that far, I doubt that is the step you were stuck at so apologies if I just explained something basic.
Try to rearrange the dy/dx into p(1+(tan4x)(tan4x))
This can then be used in the product rule (or chain, i can't remember which is which.)
Once you find that rearrange and remember y=tan4x
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