If we want to get B by itself, we need to "undo" every operation being done to it. Those operations are (in order):
- Multiplication by A
- Addition with 3C
To undo these then, we'll need to:
- Subtract 3C
- Divide by A
And, since this is an equation, we'll need to do both of those to the right side as well to keep everything equal. Here we go:
AB + 3C - 3C = D - 3C (subtracting 3C)
AB/A = (D - 3C)/A (dividing by A)
B = (D - 3C)/A
And we're done! If you'd like to see it formatted a little nicer, here's this: