tolong bantu jawab yah..!!

Maaf klo slh/krg jls..
smg mmbntu... :)

4/x = 7/(x+9)
4(x+9) = 7x
4x+36 = 7x
7x-4x = 36
3x = 36
x = 12
K = 7+17,5+21 = 45,5cm