Check the picture below.
[tex]{\LARGE \begin{array}{llll} y = ab^x \end{array}} \\\\[-0.35em] ~\dotfill\\\\ \textit{we know that} \begin{cases} x=0\\ y=40 \end{cases}\implies 40=ab^0\implies 40=a(1)\implies 40=a \\ ~\hfill \underline{y = 40b^x} \\\\[-0.35em] ~\dotfill[/tex]
[tex]\textit{we also know that} \begin{cases} x=3\\ y= 15 \end{cases}\implies 15 = 40b^3\implies \cfrac{15}{40}=b^3\implies \cfrac{3}{8}=b^3 \\\\\\ \sqrt[3]{\cfrac{3}{8}}=b\implies \cfrac{\sqrt[3]{3}}{\sqrt[3]{8}}=b\implies \cfrac{\sqrt[3]{3}}{2}=b~\hfill \underline{y = 40\left( \frac{\sqrt[3]{3}}{2} \right)^x}[/tex]
