Question

solve the equation |k-10|=3

Answer 1

if (k-10) < 0, then -(k-10) = 3
-k+10 = 3
-k = -7
k = 7

if (k-10) > 0, then k-10 = 3
k = 13

Answer 2

This is from the property of absolute value:
|x| = -x if x < 0
= x if x > 0

here, just think that (k-10) is your x, so substitute it to get |k-10|...