6r -4 +8(15-r) + 23
expand brackets
6r -4 +120 -8r + 23
collect like terms
120 + 23 - 4 = 139
6r - 8r = -2r
139 - 2r
139 - 2r = -3r + 137
-137 from both sides
-2r = -3r -2
add 3r
R = -2
sub in both equations and see if you get the same value
-3r + 137
-3 (-2) + 137
6 +137 = 143
139 - 2r
139 - 2(-2)
139 + 4
143
yes they are equal