This is a sine wave of magnitude 1.5. The value of i for any value of t is found by straight-forward sunstitution intoteh formula you gave us. Thus at t = 20 milliseconds you have:
However, you didn't tell us whether this formula yields i in amps, or milliamps. Presumbaly you can convert if need be.
The second queston involves finding the inverse of the sine function. Your post is missing an important part of the question - namely precisely what value of i you were given. So I will give an example where i = 0.75. You have:
For what value of does ? From basic triginometry you know that and . Hence:
Hope this helps.