When you are subtracting fractions do you find the common denominators

Simplify (a/b - b/a) times (a - a^2/a+b) a - a^2/a=b is a mixed number

Ineed with this problem 21 show steps

Suppose babies born after a gestation period of 32 to 35 weeks have a mean weight of 25002500 grams and a standard deviation of 800800 grams while babies born after a gestation period of 40 weeks have a mean weight of 27002700 grams and a standard deviation of 385385 grams. if a 3232-week gestation period baby weighs 21252125 grams and a 4040-week gestation period baby weighs 23252325 grams, find the corresponding z-scores. which baby weighs lessless relative to the gestation period?