Okay, so when you solve for a variable (letter) you must "move" every term on the same side with the variable to isolate the variable.
You must move the other terms by using inverse operations.
The 3/4y is positive so the inverse operation is subtraction. So, subtract 3/4y from BOTH sides.
c = 3/4y + b
c - 3/4y = b
b = c - 3/4y
Another example, r - 5x = g, solve for x.
The r is positive so the inverse is subtraction
r - 5x = g
-r = -r
0 -5x = g - r
-5x = g - r -5x means the product of 5 and x; the inverse operation is division
-5x = g - r
1x = g - r