confused about six pack?

i did a rough measurement of my body fat (rough as i used a random calculator online.. and it seemed realistic tbh) and got around 7-9% body fat.. surely this should be low enough to show off my abs..

im 17 and literally cant put on weight either, im pretty active and i basically only would eat roughly 1500 calories a day anyway

so then im guessing that i need to build muscle.. any good exercises to do this that i could incorporate weight into aswell? ive tried just doing crunches and sit ups etc before but they just dont seem to make any difference at all..

(if it makes a difference, im 5'9 and weigh around 10 stone, size 28 waist)

thanks for any help!
 
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