Australian AMC 2023 - Senior Question 15

Question 15

In the solution to this number puzzle, whenever there are three numbers in a straight line, the middle number is the sum of the other two. What is the value of $x$?

Question illustration

$\frac12(a+b+c)$
$\frac12(b-a-c)$
$\frac12(a+b-c)$
$\frac12(a-b+c)$
$\frac12(b+c-a)$