Just think about each side side separately
There a 4 isosceles triangles, all the same, and 1 square base.
Hopefully you know how to find the area of the square.
Each triangle is 1/2 x base x height (the height is the slant height of the pyramid)
If you are not given the slant height you will need to work it out using the base the perpendicular height and pythagoras' theorem.
+118654 
