Newton's law says that . Take natural logarithms of that equation: . So what you should do is to graph the logarithm of theta against t. The graph should then look like a straight line, with gradient –k and intercept . I haven't tried to check the numbers, but it looks as though that is what you have done. If so, you're right!
Once you have the values for and k, you can plug them into the original formula , and use it to find for any given value of t.