1+ { 2 × [ 3 + ( 15 - x ) ] : 7 } × 2 = 9
{ 2 × [ 3 + ( 15 - x ) ] : 7} x 2 = 9 - 1
{ 2 × [ 3 + ( 15 - x ) ] : 7 } × 2 = 8
2 × [ 3 + ( 15 - x ) ] : 7 = 8 : 2
2 × [ 3 + ( 15 - x ) ] : 7 = 4
2 × [ 3 + ( 15 - x ) ] = 4 x 7
2 × [ 3 + ( 15 - x ) ] = 28
3 + ( 15 - x )= 28 : 2
3 + ( 15 - x ) = 14
15 - x = 14 - 3
15 - x = 11
x = 15 - 11
x = 4