Help I don't understand

Solve for f over the integers:
abs(f+10) = 1

Split the equation into two possible cases:
f+10 = 1 or f+10 = -1

Subtract 10 from both sides:
f = -9 or f+10 = -1

Subtract 10 from both sides:
Answer: | 
| f = -9 or f = -11

F=-9