Help with solving a trigometric equation.

Hello,

The equation is as follows:

With the interval .

So what I did was let and since cos is negative in the 2nd and 3rd quadrants with a basic angle of .

I then continued to solve the two equations and ended up with x= and .

However I'm missing out the two angles and . I'm supposed to be adding to and right? But it doesn't give me the other two angles that I missed out.

Also, regarding intervals, when I multiply the a number into the brackets, does it change? Like for the above when I multiply the 2 to get 2x, does the interval get multiplied by two also? That is .