Since your question is "what am I doing wrong", you are making one of two possible mistakes:
One is that you are committing an "appeal to authority" fallacy. You believe the statement because you found it in your textbook. However, the statement is false: "If P then Q" and "Q only if P" are not equivalent.
The other and frankly more likely mistake is that you have misread what your textbook says. Maybe there was a section in the textbook titled "show which of the following statements are true and which are false".
A simple example where the statements are not equivalent: "If a person is a man, then that person is a human being" vs. "A person is only a human being if that person is a man".