Here's how to reduce a fraction: Break down both the numerator (top number) and denominator (bottom number) into their prime factors. Cross out any common factors. Multiply the remaining numbers to get the reduced numerator and denominator.
It went from 10 to -2, so you need to find the difference. The way you'd do this is: 10 - (-2). Do you know how to subtract a negative number from a positive number?