For part a), when an equation crosses the x axis, the y coordinate is zero, hence the equation becomes:

Now, when you take the inverse sine of 0 you get an infinite number of solutions, but only one of them lies in the interval . Find that solution, then it's just a case of solving algebraically for x.

For part b it is the exact same process except that you need to find take the inverse sine of and find the solution to it which lies in the given interval.