SIGNIFICANT DIGITS THE ANSWER IS NOT −16.422

Of course it is not -16.422 because you are warned about significant digit.

The result is -16.422000 but if you consider the significant digits, the answer is -16.

That's right.

The greatest number in the multiplication process is the multiplier (5.1)

Now how many significant digits does 5.1 have? 2 significant digits (5.1)

Now the answer should also have 2 significant digits.

-16.422

Take the first 2 digits from the left.

-16.422

The underlined number is the product of the two numbers, in terms of significant digits.

c. newton's third law of motion