a. 23 or -23
This is because how far x is away from 0 on a number line is it's absolute value which is represented by the bars.
b. 2.3
Same thing here. x is 2.3 because it's absolute value would have to be 2.3, which is what the answer says. (The negatives cancel each other out.)
c. 9
x is 9, so that 9+1 is 10, which is also ten in absolute value language here, and then that's the answer, too, so it all fits.
d. 3
Simplify the expression as k+4=7, and there ya go.