i)
1 foot is a larger unit that 1 inch,
exactly: 1 foot = 12 inch.
ii) we add same units, that is we add feet to each other, and inches to each other:
5 feet 9 inches + 12 feet 3 inches = (5+12) feet +(9+3) inches
=17 feet 12 inches
iii)
since 12 inches = 1 foot,
17 feet 12 inches = 18 feet.
Answer: 18 feet