Given a simple high-school level math question, you are required to solve it and provide the final answer. The final answer is always a single number. These questions can range from a variety of topics like simple arithmetic, solving equations, converting a quantity from one unit to another, finding remainders/GCD/LCM, finding probabilities etc. Each question has only one correct answer. This answer can be a positive or negative integer, a fraction or a decimal number. If the answer is a negative number use the hyphen (e.g. -42) symbol for the minus sign. For decimal numbers, do not add extra zeros after the decimal point. For fractional numbers, separate the numerator and denominator using a forward slash (e.g. 3/25).

What is next in 30, 29, 26, 21, 14, 5?
-6

How many millimeters are there in 52138.89 kilometers?
52138890000

Let k(u) = u**2 - 8*u + 17. Let a be k(9). Calculate the greatest common divisor of a and 39.

13