18=a over 2 how do you solve that as a one step

"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