$100h + 10t + u$ = the number
The hundreds digit is twice the units digit
$h = 2u$ ← equation (1)
The sum of the digits is 17
$h + t + u = 17$ ← equation (2)
396 be subtracted from the number
$(100h + 10t + u) - 396 = 100u + 10t + h$
$99h - 99u = 396$
$h - u = 4$ ← equation (3)
Substitute h = 2u to equation (3)
$2u - u = 4$
$u = 4$
From equation (1)
$h = 2(4)$
$h = 8$
From equation (2)
$8 + t + 4 = 17$
$t = 5$
The number is
$100h + 10t + u = 854$ answer