I am not familiar with the 84, but I have an 83 Plus.
With it, all you have to do is store 5 in variable X. Then, as mmm said, type in X^2-3X+5 and it will return 15.
X is a 2nd function on the 83. You hit ALPHA STO to display X.
To store 5, or any other numerical value, in the variable X...