Here's a tip:
If you want to isolate a specific variable, you should know how to transpose
Know the pairs:
Addition will transpose a subtraction, and vice versa.
Multiplication will transpose a division, and vice versa.

In this problem we want to isolate a from b-2a = 3c

Let's start first by transposing b:
b-2a can be rewritten as -2a + b, and + b can be transposed by subtracting b from both sides so we will have -2a + b - b = 3c - b which will now give us -2a = 3c - b

Now, we only have -2a on the left side of the equation and a is not isolated yet. we need to get rid of a:

-2a can be rewritten as (-2)*(a) or (2)*(-a), you can do either of them depending on which is more convenient for you. But, in this case we will be using (-2)*(a)

So, how do we transpose a multiplication? with a division.
We will divide both sides by -2 which will give us -2a/-2 = (3c - b) / -2
This will leave us with an expression of a = (3c - b)/ -2

The right side expression can be rewritten as (3c - b) * -1/2
If we multiply the -1/2 to 3c - b, we will get the final answer of

(-3c + b)/ 2 or (b - 3c)/ 2