White has an open three: either end becomes an open four next move, and an open four makes five no matter what black does. Against it you must take an end — a middle block leaves both four-completions open — and when white converts to a four anyway, it threatens exactly one five point; hold that one, and the three is dead. The one alternative to blocking is racing: if black's own four lands first, the three becomes irrelevant — which is why you always check your own forcing moves before answering the opponent's threats. Now you defend, same shape, your stones: white's three on row 7 is live. Cap it.
Finish this step to move on.