Respuesta :

given that C is the midpoint of AB so from this result that AC = BC so given that AC = 20 => BC = 20 too 
and given that AB = 5x so AB = AC +BC so AB = 20+20 so AB = 40 
than AB = 40 and AB = 5x so result 5x = 40 => x = 40/5 so x = 8

hope this will help you