are there exceptions for the multiplying top by bottom or can i always do that i understand that i may have to input a negative sign somewhere example   4x - 3y = -135x +2y=1now i * i use 4 or 5

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You can always do it though sometimes it creates larger numbers than you may want to work with.  One way of thinking about it is to take the 2 numbers (from top and bottom) and pretend they are denominators and find the common denominator then multiple each by what it needs to become the common denominator.With 4 and 5 the common denominator would be 20, the same as it you just multiplied them together but what if you had the problem:4x  -  5y = 2510x + 3y = 15you can multiply the top by 10 and the bottom by 4 or find the "common denominator" for 4 and 10 which is 20.  The top would then be multiplied by 5 and the bottom by 2.  Smaller numbers to work with.  Both ways will get you the correct answer.