There are a few ways. If you know calculus, take the derivative and find the critical points.
If you don't know calculus. You can also graph the equation either by hand with several test test points or with a graphing utilitiy.
If you prefer to not graph it, you can do some critical thinking. For example, on the interval from x=0 to x = pi/2, you have 3sinx going from 0 to 3. On this same interval you have cos2x going from 1 to -1. Thus you can determine which one changes value faster. Then use that information to figure out at what angle the fastest growing function stop growing.