Abox contains a collection of triangular and square tiles. there are 25 tiles in the box, containing 84 edges total. how many square tiles are there in the box?

There are 25 tiles total - triangles or squares - 84 edges
 
triangle (t) = 3 sides
square (s) = 4 sides

t+s= total tiles 
t+s = 25 ← solve
3t + 4s = total sides
3t + 4s = 84 ← solve

t = 16   s = 9

Check

16 + 9 = 25

3(16) + 4(9) = 84
48 + 36 = 84

There are 9 square tiles in the box