Example:

Look at the denominator of the fractions, and then see whether there is a common prime number associated with them.
In this example, since 5 and 9 (3*3) do not have any common prime numbers, you just multiply these numbers together and get 45.
There, we just found the LCD of fractions.
To find angles you have to use an inverse function. Tan-1(25/45). your angle is 29.0546041 but rounded to the nearest hundredth it is 29.05.
12=-9s+7s
12=-2s
-6=s
Hope this helps