Police are asking for the public’s help in finding the driver of a car involved in a hit-and-run in which a 68-year-old woman was critically injured Wednesday afternoon. Luyiza Polak was hit by a sport utility vehicle just after 5 p.m. at 500 Lynnfield St., police said. Polak got off an MBTA bus and was struck as she crossed Lynnfield Street, heading toward North Shore Medical Center’s Union Hospital campus, Lieutenant Christopher Kelly said in a statement. The SUV fled toward Lynnfield. Polak was taken to Massachusetts General Hospital in Boston. The hospital did not immediately return a call seeking her condition.