Question 80234
You will need to get a common denominator.  It will have to be 4x.

To make 3/x have a denominator of 4x, you will have to multiply by 4 and get (3*4)/4x = 12/4x.  
To make 1/4 have a denominator of 4x, you will have to multiply by x to get
x/4x.
To make 1/x have a denominator of 4x, you will hav to multiply by 4 to get
4/4x.

Your problem is now 12/4x - x/4x = 4/4x.
On the left, subtract numerators and keep the same denominator.  Your problem is now (12-x)/4x = 4/4x.  Now multiply by 4x to get rid of the denominators.  You now have 12 - x = 4.  Add x to both sides to get 12 = 4 + x.  Subtract 4 from both sides and you will get x = 8.