You are given two equalities:

 = ,    (1)   and
 = .   (2)

From (1):  a*ln(3) = b*ln(5),                          (3)   and

From (2):  b*ln(5) = c*ln(75) = c*(ln(3) + 2*ln(5)).   (4).    ( <--- since 75 =  )

From (3), express ln(3) =  and substitute it into (4). You will get:

b*ln(5) = ,  or

ab*ln(5) = bc*ln(5) + 2ac*ln(5).

It implies

ab = bc + 2ac,  or

ab - c*(2a + b) = 0.

Answer.  ab - c*(2a + b) = 0.