Answer:
A
Step-by-step explanation:
We want to solve the equation: 

To do so, we can rewrite the equation. 
Recall the double-angle for sine:

By substitution: 

Distribute: 

We can subtract 3cos(x) from both sides: 

And factor: 

Hence, our answer is A. 
*It is important to note that we should not divide both sides by cos(x) to acquire 10sin(x) = 3. This is because we need to find the values of x, and one or more may result in cos(x) = 0, and we cannot divide by 0. Hence, we should subtract and then factor.