1/2 - x/30 = 3(x + 7)/90.

 1      x     3(x + 7)
--- - ---- = ----------
 2     30        90

2, 30 and 90 will all divide evenly into 90. So 90
is the LCD.

Put 90 over 1 like this with a times sign beside it.

 90
----·
  1

Now stick it before every one of those fraction terms:


 90   1     90    x     90   3(x + 7)
----·--- - ----·---- = ----·----------
  1   2      1   30      1      90

Now you can cancel the 2 into the 90 on the left, 
getting 45. You can cancel the 30 into the 90, 
getting 3.  You can cancel the 90's on the right.

 45         3           1    
 90   1     90    x     90   3(x + 7)
----·--- - ----·---- = ----·----------
  1   2      1   30      1      90
      1          1              1

Now all the denominators are just 1 and therefore can be
left understood, so we just have this::

             45 - 3x = 3(x + 7)

Now you can finish that, right?

Answer:            x = 4

Edwin