Explanation:
first combine like terms : 6r and -7r = -1r
21=5 -1r
cancellation cancel the same terms on both sides of the equation by inverse operations:
subtract 5 from both sides 21-5= 16 5-5=0
16=-1r
divide both ends of the equation by -1
16÷1 = 16 1÷ -1= 1
16=r