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
CREATE TABLE "specific_fuel_consumption_sfc_specific_i" (
  "engine_type" text,
  "scenario" text,
  "sfc_in_lb_lbf_h" real,
  "sfc_in_g_k_n_s" real,
  "specific_impulse_s" real,
  "effective_exhaust_velocity_m_s" real
);
SELECT AVG("specific_impulse_s") FROM "specific_fuel_consumption_sfc_specific_i" WHERE "sfc_in_lb_lbf_h"=7.95 AND "effective_exhaust_velocity_m_s">'4,423';