minta tolong dijawab ya....

s = 12 cm, nyata
s' = 18 cm, nyata (ditangkap layar)
f = __?

1/f = 1/s + 21s'
1/f = 1/12 + 1/18
1/f = 3/36 + 2/36 = 5/36
f = 36/5 = 7,2 cm ✓

jarak fokys cermin 7,2 cm.