<prompt> Hebu px 2x - 7 na qx 3x - b. Kama pq4 7, nini b?  <response>: Tangu q4 3 4 - b 12-b, tunaweza kuandika pq4 7 kama p12-b 7. Tangu px 2x-7, tuna p12-b 212-b - 7 17 - 2b. Substitute hii katika p12-b 7 inatoa 17-2b 7, ambayo tuna b 5. 