Question

Thank you to whoever decides to help :)

Answer 1

A. r - s - t

Explanation:

Observe the bounds of each integral. (Think of it as a number line if it's confusing.)
r is [1, 4]

s is [1, 2]

t is [3, 4]

Name the integral from [2, 3] = u.

s + t + u = r

u = r - s - t