I'm going to assume that you meant y = mx + b not y = my + b
To do this move 7x to the right side by subtracting it to both sides
(7x - 7x) - y = 8 - 7x
0 - y = 8 -7x
-y = 8 - 7x
Y still isn't full isolated! There is still the negative sign that you have to get rid of. To do this divide -1 to both sides
-y/-1 = (8 - 7x)/ -1
y = -8 + 7x
y = 7x - 8
Hope this helped!
~Just a girl in love with Shawn Mendes