There are two general rules that must be followed when transposing.
(a) THE RULE OF OPPOSITES  Which indicates the appropriate operation to 'move' a symbol.
(b) THE RULE OF BALANCE  Which governs the way in which the operation is carried out.
To apply rule (a) you need to be familiar with opposite, or INVERSE, operations. Here is a list of those most commonly used:
OPERATION 
INVERSE OPERATION 
Addition (+) 
Subtraction () 
Multiplication (x) 
Division (÷) 
Square (a²) 
Square Root (√) 
For example in the formula: s = a + b the inverse of +b is b
We can therefore, 'remove' the b by subtracting b from both sides of the formula.
So we get
Step 1: s  b = a + b  b
Step 2: s  b = a
Step 3: a = s  b
To apply rule (b) you should remember that a formula remains valid, or 'in balance', provided any operation is applied equally to both sides of the formula.
For example, if P = VI
then P + Q = VI + Q
or √P = √VI
You'll notice that in each case, both sides of the formula were treated in the exactly the same manner.
Example
Make I the subject of the formula V = IR
To remove R, which is multiplying I, we must divide both sides by R:
V = IR
R R
Hence:
V = I
R
(Note that 'R' cancels on the right hand side. In practice it is simply excluded).
