C3 Rcosalpha - coordinates where graph crosses the axis? How do I do it?

How do I do part B? I don't have a clue how to do it, and I can't find any videos on YouTube either.

For part a, I got 6.5cos(x-0.395), which I guess would help slightly but I don't know how that links in either.

Please can someone walk me through it? Thank you

differentiate y and set it to 0 and find x coordinates?

Just do what you always did at GCSE: set $y=0$ and then $x=0$. Use the original form for the exact answers.

$x=0 \Rightarrow y=6 \cos(0)+2.5 \sin(0)$ and this gives the y-intercept.

$y=0 \Rightarrow 0=6\cos(x)+2.5\sin(x)$ will give the x-intercepts. [HINT: You can get tan(x) from this form, then easily solve for the two x-values in range, but you can also use your R-alpha form too here (if you prefer) since you'll round anyway and don't need the exact answer]

Okay, so I replace my 6.5cos(x-0.395) with the 6cosx+2.5sinx = 0, so then I have 6.5cos(x-0.395)=0, and I then divide by 6.5 to get cos(x-0.395)=0. How do I then solve that (to get the two coordinates)??