subtract these two rational expressions

(2x+3)/(x-1) - (x-1)/(x+1)

You need the same denominator for both fractions before you can subtract one from the other. Multiply top and bottom of the first one by the denominator of the second one, and multiply top and bottom of the second one by the denominator of the first one. You can then combine the pair of them into a single fraction with (x-1)(x+1) as the denominator. {Don't forget when you're simplifying the result that (x-1)(x+1) = (x²-1)}