15,342 rounded to the nearest thousand

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

Ineed to know the answer to this algebra question.

John used linear combination to solve the system of equations shown. he did so by multiplying the first equation by -3 and the second equation by another number to eliminate the x-terms. what number did jonas multiply the second equation by? 4x-6y=23x+5y=11