To start, write a system of equations in the form $y =ab^{x}$ by substituting the points in for $x$ and $y$.

$16.875 = ab^{3}$

$7.5 = ab^{1}$

Divide the 2nd equation into the first to eliminate a.

$2.25 = b^{2}$

Solve for $b$.

Substitute the $b$ value you found into one of the original equations.
Solve for $a$.
Substitute both $a$ and $b$ back into the original equation with along with $x$ and $y$.

$y = 5\left(1.5\right)^{x}$

$y = 0.5\left(0.4\right)^{x}$