"a" is being divided by 2.
You want a alone, not divided by 2.
The operation that is opposite to division is multiplication.
Multiplication undoes division.
Multiply a/2 by 2 to get a.
A rule of equations is that you must do the same operation to both sides of the equation, so you must multiply both sides by 2.
18 = a/2
18 * 2 = a/2 * 2
36 = a
a = 36