Hello!
First, a word to the wise: Please type x2– 3x + n as x^2– 3x + n. The " ^ " denotes exponentiation, whereas x2 is meaningless.
To make x^2– 3x + n into a perfect square trinomial, take half of the coefficient of x (that is, half of -3) and square the result:
x^2– 3x + (3/2)^2, or x^2– 3x + 9/4. The desired value of n is then 9/4.