How would I solve this question?

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You have three equations to use:

(1) [MATH]\cos{3 \theta}=4\cos^3{\theta}-3\cos{\theta}[/MATH](2) [MATH]r^3\cos{3\theta}=-4q[/MATH](3) [MATH]3r^2=4p[/MATH]
Multiply the first equation by [MATH]r^3[/MATH] and then use equations (2) and (3) to substitute into this, and see what happens.
 
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