i need help figuring out something about subtracting mixed numbers

i have been stuck on this matter for hours trying to figure it out so i can get ready for intermediate algebra at a local community college's placement test, could anyone help me figure it out?

problem: in a mathmatical problem where i subtract 1 or 2(+) mixed number(s) fraction could i find the LEAST common denominator by following a trail of common denominators before the problem is solved as an attempt to narrow it down to what the least common denominator would be or does finding the least common denominator become an absolute/default (by default i mean cannot be broken down and has only one least common multiplier that works period) must in order to solve the problem?