Answer:
Fast and loose:
If you stare at it for a while, you can see that the LHS is the distance between k and 10. Basically, you've asked to pick k so you get two numbers that are 3 apart from 10. And these two numbers are 7 to the left, and 13 to the right.
Rigorously
Split it in 2 situations, depending if the quantity inside the absolute value is greater or smaller than zero.
If it's greater or equal than zero:
And since 13 is indeed greater than 10, it's a valid solution, so you get the first one. For the second, you assume negative values for the quantity inside, and change sign:
In this case, 7 is a valid solution too since it's less than 10