For clarity, please enclose the fractional coefficients inside parentheses:
(4/15)x^3, (5/ 12)x^2
Here we have 2 different denominators, 15 and 12. To get started, factor each: 15 = 3*5, and 12 = 4*5. Thus, 5 is common to both 15 and 12. To form the LCD, begin with 5 and include the factors 3 and 4: 5*4*3 = 60.
Multiply both (4/15)x^3 and (5/ 12)x^2 (separately) by this LCD (60), and then divide the results by 60:
(4/15)*60*x^3 (5/12)*60*x^2
------------------ and ---------------------
60 60
This results in
16x^3 25x^2
-------- and ---------
60 60
These fractions are the same as the original ones, except now they share the same denominator (60).