Compute $0.18\times 12$ and then subtract that result from $5.30$ to get a numerical value for $\ln Q$.

Once you have $\ln Q =$ (some number), remember that $\ln$ is a logarithm with base $e$. What operation reverses taking a natural log so you can get $Q$ itself?

After you rewrite $Q$ in terms of an exponential expression, use the $e^x$ or exp button to evaluate it, then round to the nearest whole number.

In Desmos, type exp(5.30 - 0.18*12) or e^(5.30 - 0.18*12) as a single expression. Read off the numeric output that Desmos gives you, then round that value to the nearest whole number to match one of the answer choices.

The model is

We want the amount of medication after 12 hours, so substitute $t=12$:

Now calculate $0.18\times 12$.

Evaluate the product:

So

This means the natural log of $Q$ after 12 hours is $3.14$.

The equation $\ln Q = 3.14$ means that $Q$ is the number whose natural log is $3.14$. To undo a natural log, exponentiate both sides with base $e$:

Now we just need to approximate $e^{3.14}$.

Use a calculator for $e^{3.14}$ (or the exp(3.14) function):

Rounding $23.1$ to the nearest whole number gives $Q \approx 23$ milligrams.

So, the predicted amount of medication remaining after 12 hours is 23 milligrams.