Answer
The sum of both blanks has to be 9
Work Step by Step
x=9
$9 + a = 2 \times 9 - b$ a and b are the blanks
$9 + a = 18 - b$
$a + b = 18 - 9$
$a + b = 9$ if you add up the blanks the sum has to be 9
These are all possible equations:
9+8=18-1=17
9+7=18-2=16
9+6=18-3=15
9+5=18-4=14
9+4=18-5=13
9+3=18-6=12
9+2=18-7=11
9+1=18-8=10
9+0=18-9=9
