For this case what you should do is evaluate values of x in the function and verify that they meet the result of f (x) shown in the graph.
The answer is
f (x) = - 2lxl +1
notice that
f (1) = - 2l1l + 1 = -1
f (-1) = - 2l-1l + 1 = -1
Both comply with the value of f (x) shown in the graph
answer
f (x) = - 2lxl +1