Yg Nomer 17 caranya gimana?

p : l : t = 8 : 6 : 5

p = 8x
l = 6x
t = 5x

luas diarsir = l . (p+t)
312 cm² = 6x . (8x+5x)
312 cm² = 6x . 13x
312 cm² = 78x²
x² = 312 cm² / 78
x = √ 4 cm²
x = 2 cm

p = 8 . 2 cm
= 16 cm

l = 6 . 2 cm
= 12 cm

t = 5 . 2 cm
= 10 cm

luas jaring jaring Balok :
= 2 . p . t + 2 . p . l + 2 . l . t
= 2 . 16 cm . 10 cm + 2 . 16 cm . 12 cm + 2 . 12 cm . 10 cm
= 320 cm² + 384 cm² + 240 cm²
= 944 cm²