4k+mn=n-3; solve for n

Since we are solving for n you have to isolate the n. Therefore, you want all the variables with an n in it on one side and all the variables without an n on the other side:) :
4k+3= -mn+n (There were a change in signs because for example you moved the -3 to the other side, therefore it can only be a negative on one side so now you have to change the sign to a positive:) hope you got that)
That would've been your answer because you cannot do anything else...Hope this helped :)