you make the denominators the same by finding the common denominator. to find the common denominator, multiply the denominators on both sides. Now you have the denominator of 45. next you multiply 5 times 3 (the top of one side and the bottom of the other). then multiply 9 times 4. subtract the product of those from each other and you get 21. since 45 is your denominator, you are left with 21/45.
