how do i calculate the slant height of a square pyramid? is there like a formula or something??

It depends on what info you have but you can use the pythagorean theorem or trigonometry.

We need to know the height and the length of a side.

The slant height is given by

√[(1/4)s^2 + h^2]    where s is the side length and h is the height

Alternatively...if we know the height and the angle  - θ  -  that the side of the pyramid makes with the base along one edge [ not at a vertex, though!!! ], we can use

slant height = height / sin θ