The problem asks for a demand function based on price and quantity, and to graph it.

From the information:

• When the price $P = 20,000$ IDR, the demand $Q = 2$ units.
• When the price $P = 18,000$ IDR, the demand $Q = 3$ units.

We can express the demand function in the form of a linear equation: $P = mQ + c$ where $P$ is the price, $Q$ is the quantity demanded, $m$ is the slope, and $c$ is the intercept.

Step 1: Find the slope (m)

The slope of a line is calculated using the formula: $m = \frac{P_2 - P_1}{Q_2 - Q_1}$ Substitute the given values: $m = \frac{18,000 - 20,000}{3 - 2} = \frac{-2,000}{1} = -2,000$ Thus, the slope $m = -2,000$.

Step 2: Find the intercept (c)

We use one of the points to find the intercept. Let's use $P = 20,000$ and $Q = 2$: $20,000 = -2,000(2) + c$ $20,000 = -4,000 + c$ $c = 24,000$ Thus, the demand function is: $P = -2,000Q + 24,000$

Step 3: Rearrange to the demand form

The demand function is typically written with $Q$ as the subject: $Q = \frac{24,000 - P}{2,000}$

Graphing the Function

The demand function $P = -2,000Q + 24,000$ is a straight line with a negative slope. It starts at $P = 24,000$ when $Q = 0$ and decreases by 2,000 for each additional unit of $Q$.

If you'd like, I can graph this function for you.

Would you like more details on the process? Here are some related questions:

1. How do we interpret the slope in a demand function?
2. What does the intercept represent in the context of this problem?
3. How would the demand curve change if the slope was steeper or flatter?
4. How can this function help in predicting future demand based on price changes?
5. How would we calculate elasticity from this demand function?

Tip: The slope in a demand function represents how sensitive the quantity demanded is to changes in price. A steeper slope means demand is less sensitive to price changes.