To convert any unit to it's "milli-" form always multiply the given value by 1000. The new value will be the value of in the "milli-" form.
The question asked to us is to convert 3.521 g to mg (or milligram). Thus, all that we need to do is to multiply 3.521 with 1000 and we will get the corresponding value in milligrams. Thus, in mg, 3.521 g is:
mg