To subtract fractions with differen denominators you must convert to equivalent fractions with the same denominator.
The best practice is to convert to fractions with the least common denominator.
Given that 8/36 is not the simplest form, you can start to simplify it, by dividing both numerator and denominator by 4:
8/36 = 2/9
Now find the least common denominator of 9 and 14. That is found by decomposing the numbers and taking the product of the common and not common prime factors each raised to the its greates power.
9 = (3^2) 14 = (2)(7)
Therefore the least common denominator is: (2)(3^2)(7) = 126
So, convert both fractions to the equivalent fraction with denominator 126:
2 * 14 28 ---------- = ---------- 9 * 14 126
3 * 9 27 -------- = -------- 14 * 9 126
Now you subtract the two fractions with the same denominator:
