After you substitute $p=6$, simplify $4(6)$ and then move that result to the other side of the equation to get an equation with only $c$ on the left side.

Once you have an equation like $0.5c = \text{(some number)}$, remember that dividing by $0.5$ is the same as multiplying by 2.

In a new Desmos expression line, type (60 - 4*6)/0.5 and press Enter. The output is the value of $c$, the number of postcards that can be printed in the same amount of time as 6 posters.

We are told the shop produces $6$ posters, so substitute $p=6$ into the equation $4p + 0.5c = 60$:

This represents the total printing time when 6 posters and $c$ postcards are produced.

First, multiply $4(6)$:

Now isolate the $0.5c$ term by subtracting 24 from both sides:

Now solve $0.5c = 36$ by dividing both sides by $0.5$:

Dividing by $0.5$ is the same as multiplying by 2, so $c = 72$.

Therefore, the shop can produce 72 postcards, which corresponds to answer choice D.