"im a 21 years old guy and I weigh 143 lbs with 70.9inch tall. I want to gain weight but my appetite doesn't help. Any suggestions? "
Why underweight? There are lots of reasons for being underweight, & these should be ruled out by medical evaluation. Assuming your work-up identifies no maldigestion, malabsorption, inflammatory disease, chronic infection, metabolic abnormalities, ca, or non-gi systemic disease then weight gain is a simple matter of taking in more calories than you burn. Supplement with fortified milk shakes/ commercial products.
Becomephysically fit. Assuming no health problems: A pound of fat takes up more than 3 times the volume of a pound of lean muscle. So the fastest way to gain weight in a healthy manner is to become physically fit--while eating a healthy diet. An appropriate fitness program will not make you suffer--muscle aches and pains are not necessary. Just 11 minutes a day 5 days a week with no discomfort.