$100h + 10t + u$ = 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*