We know that
Density = mass / volume
And volume = L*W*H (thickness)
First calculate the volume of foil as follows:
Density = mass / volume
Volume = mass / density
= 2.722 g/ 7.26 g/ cm^3
= 0.375 cm^3
And volume = = L*W*H (thickness)
0.375 cm^3 = 5cm*5 cm * thickness
Thickness = 0.015 cm