-67 = -4n + 3(1 + 6n)
-67 = -4n + 3 + 18n
-67 color(red)(-3) = -4n color(red)(cancel(color(black)(+ 3) -3)) + 18n
-70 = -4n + 18n
-70 = 14n
-70/14 = (color(red)(cancel(color(black)(14)))n)/color(red)(cancel(14))
n = -70/14
color(blue)(n = -5
Now we can substitute n for -5 to prove our answer.
-67 = -4 xx -5 + 3(1 + 6 xx -5)
-67 = -4 xx -5 + 3(1 + -30)
-67 = -4 xx -5 + 3 xx -29
-67 = 20 + 3 xx -29
-67 = 20 + -87
-67 = -67