remember that you can do anything to an equation as long as you do it to both sides except divide by zero
so you can add like terms like this
you can add similar things like 1+3 and m+2m and m^2+2m^2 but not unlke things like 1+m or m+m^2
so m+2m=1m+2m=3m so
r-4+6r=3+8r
use commutative property to move numbers around (a+b=b+a)
group like terms
r+6r-4=3+8r
add like terms
1r+6r-4=3+8r
7r-4=3+8r
subtract 7r from both sides
7r-7r-4=3+8r-7r
0r-4=3+1r
-4=3+r
subtract 3 from both sides
-3-4=3-3+r
-7=0+r
-7=r
r=-7
