Mark M. answered 11/20/20
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Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
d/dx(axb) = abxb-1
abxb-1 = 5bxa-1 when ab = 5b and b-1 = a-1.
Since b is not zero, we get a = 5 and b = 5.