Trig functions and equation

i need help with the question in this link...
im not sure how to work it out..

https://imgur.com/a/rIB0A

i need help with the question in this link...
im not sure how to work it out..

https://imgur.com/a/rIB0A

(a) All you need to do is firstly express the equation entirely in terms of $\sin \theta$, then just use the given substitution $x= \sin \theta$ to obtain what they have.

(b) (i) That's just a quadratic to solve, I'm sure you can do this.

(ii) Use the fact that we denoted $x = \sin \theta$, and along with the solutions to (i) to then solve for $\theta$ in the given domain.

$3\cos^2 \theta = 3(1-\sin^2 \theta)$

and when you expand that you get

$3 - 3\sin^2 \theta$

You just wrote $3\sin^2\theta$ in your working.