Unfortunately you cut off the part of the problem that gives the coefficient of t. Are you using the "," to separate whole and decimal parts (European notation) or to separate thousands (American notation). In other words is that coefficient "one and three hundred eighty six thousandths" or is it "one thousand three hundred eighty six"?
If it is the latter, you are way off. Do the whole problem again. If it is the former, you have misplaced the comma in the answer. As for writing it to "three significant figures" what do you understand "significant figures" to mean?
I don't understand your question in (2). "How can I know in (some symbol) demand N or N0". I might guess that "N" means "natural numbers" (positive integers). Does "N0" mean "whole numbers" (non-negative integers).
If you are asking whether the domain is intended to be all positive integers or all non-negative integers, the answer is "neither"! The default domain for an exponential function is all real numbers- both positive and negative, and fractions, etc.
I would recommend using at least two or three negative numbers, as well as 0 and three or four positive values. (For t= 5 you seem to have confused "," and "." again.)