PLS HELP WILL MARK YOU BRAINLIEST, NO FAKE ANSWERS!!

What are “like terms”? why can we only add like terms?

What is the missing step in this proof ?

Which values of p and q result in an equation with exactly one solution? px-43=-42x+q choose all answers that apply: a) p=-43 and q=-43 b) p=42 and q=43 c) p=43 and q=−42 d) p=42 and q=42