Respuesta :
12 = f - 13 - 2
12 = f - 15
12 + 15 = f - 15 + 15 ... add 15 to both sides
27 = f + 0
27 = f
f = 27
12 = f - 15
12 + 15 = f - 15 + 15 ... add 15 to both sides
27 = f + 0
27 = f
f = 27