question: what is the average specific impulse for engines that have a SFC in lb/(lbf·h) of 7.95, and a Effective exhaust velocity (m/s) larger than 4,423
answer: SELECT AVG(specific_impulse__s_) FROM table_name_66 WHERE sfc_in_lb__lbf·h_ = 7.95 AND effective_exhaust_velocity__m_s_ > 4 OFFSET 423
context: CREATE TABLE table_name_66 (specific_impulse__s_ INTEGER, sfc_in_lb__lbf·h_ VARCHAR, effective_exhaust_velocity__m_s_ VARCHAR)