One way to do solve this problem:
-- find f(-6) and f(-5); if one of these answers is positive and the other is negative, the zero will be between
these two values
-- if not, find f(-5) and f(-4); if one of these answers is positive and the other is negative, the zero will be
between these two values
-- if not, find f(-4) and f(-3); ...
-- if not, find f(-3) and f(-2); ...