Essentially, you’re solving/isolating the variable you want to be the subject. So, since we want x as the subject, we’ll solve for x.
For example. let’s take the first one. We want to make x the subject. So, we’ll solve for x.
x+h=4
Subtract h from both sides
x+h-h=4-h
The h’s cancel on the left side, so we’re left with only an h on the other side.
x=4-h
The variable x is now the subject, as the equation is arranged so that the variable x equals something.