If you rearrange a formula, you change the variable which is its subject. In A = b × h (formula for area of a rectangle), A is the subject, because it's own its own on one side of the equal sign.
You can change the subject of the formula by using inverse operations. Have a look at an example below.
v = u + at (formula for final velocity). Make a the subject of the formula.
We need to inverse the operations. Let's first move u to the other side by subtracting:
v - u = at
at means a × t. So reverse this, we need to divide by t.
(v - u) ÷ t = a
Now a is the subject of the formula. It is on its own on one side of the equation.
See how (v - u) are in brackets? This is because we want to divide the entire left-hand side by t, not just v or u.