Need some help with solving equations

I am having a lot of trouble with these questions.

6. 5(sin 6t - 4cos 4t) = 0

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10. a) 3sin^2x - 8sinx - 3 = 0

This one I kinda get, but I am having some trouble with it.

The factors can be found like this right?

so therefore ... 3sinx + 1 =0 and sinx - 3 =0

For 3sinx+1=0, you move 1 over, so it becomes 3sinx = -1

You then divide by 3 so it equals sinx = -1/3

it then becomes x = sin^-1(-1/3) but that's as far as I know how to do it.

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I have the answers to these, I just need to know how you get each step to solve the answer.

**Also, I have a question concerning angles. For example, would -20 deg, be the same as 20 deg and how so? **