This is cross multiplying, so basically you're multiplying in a big X. So,
multiply (1/2) by 4, to get 2, and then multiply 3 by x, to get 3x.
You now have 2 = 3x, and we need to isolate the variable. To do that, we use the opposite operation the "x" is involved in. Here we have 3x (the x is multiplied to the 3) so we will divide 3x by 3. This cancels out the 3, leaving just "x".
What you do to one side of the = sign, you must do to the other, so we will also divde 2 by 3. This will give you 2/3 = x or, 0.67 = x (whichever form your answer must be in).
I know it can be confusing, so I including a picture working it out to help. Good luck!