Let X be the point where the external tangent meets circle A.
Let Y be the point where the external tangent meets circle B.
Because AX is perpendicular to CYX and BY is perpendicular to CYX, triangle(ACX) is similar to triangle(BCY).
Therefore: CB : CA = BY : AX.
Since CA = CB + BA = CB + 17 and BY = 7 and AX = 10:
---> CB : CB + 17 = 7 : 10
---> 10 · CB = 7 · (CB + 17)
Can you finish it from here?