We have:
[tex]6 = ab^3[/tex]
[tex]24 = ab^5[/tex]
If you divide the second equation by the first:
[tex]\dfrac{24}{6} = \dfrac{ab^5}{ab^3}[/tex]
[tex]4 = b^2[/tex]
b = 2 (we discard the negative solution because in the exponential function the base must be positive.)
Then:
[tex]6 = a\cdot 2^3[/tex]
[tex]6 = a\cdot 8[/tex]
[tex]a = \dfrac{6}{8}[/tex]
[tex]a = \dfrac{3}{4} = 0.75[/tex]
