Question

How do I find xf in the equation
xf= 1/2at^2 + vit+ xi

Answer 1

there are 4 unknowns in the right hand side of the equation

You need to read the whole question and see if it tells you the values for

a (=acceleration)
t (= time elapsed)
vi (= initial speed)
xi (initial displacement)

If so then put those values into the equation and solve for xf (= final displacement)

Note that if it a body falling or thrown under gravity then you can assume a=g=9.81m/s^2

Answer 2

Also - if the question asks something like 'how far has it travelled after time t' then you can assume you are measuring distance from the position it started at - so you can set xi=0