Surface area new bubble = surface area of first bubble + surface area of second bubble ⇒‌‌A=A1+A1 ∵ Surface area of a bubble, A=8πR2 ∴‌‌8πR2=8πR12+8πR22 ⇒‌‌R2=R12+R22 (give that R1=3mm,R2=4mm) ⇒‌‌R2=(3)2+(4)2 ⇒‌‌R=5mm