jawabnya pakai caranya lengkap dengan gambarnya ...

k = 4 x s
40 = 4s
s = 40/4
s = 10cm

panjang 1/2 diagonal
= 16/2
= 8cm

10² = 8² + d²
100 = 64 + d²
d² = 100 - 64
d² = 36
d = √36
d = 6 cm

panjang BD
= 6 x 2
= 12cm