Determine the demand function if, at a price of 5 rubles, the seller is ready to sell 100 pieces of goods

Determine the demand function if, at a price of 5 rubles, the seller is ready to sell 100 pieces of goods, and if the price increases to 10 rubles, he sells 50 pieces of goods.

The demand function has an equation of the form y = kx + b, where x is the price P, y is the volume Q.
To find the demand function, we will compose a system of equations based on our data:

5k + b = 100
10k + b = 50

Let us express b from the first equation:
b = 100-5k

Substitute this into the second equation and find k:
10k + (100-5k) = 50
10k + 100-5k = 50
5k = -50
k = -10

Find b:
b = 100-5 * (- 10) = 150

Then the demand function has the form: Q = 150-10P