Hello! We can see this equation as a function like f(x) = 5x + 3 = 15
Let's think about a problem...
For example, we have a taxi that charges $5 per kilometer + a flat fee of $3.
So, we can write it as a function:
f(x) = 5x + 3
Look that x = kilometers, so this value will grow according to the amount, right?
Knowing that, let's go back to your equation, but thinking in the taxi:
5p + 3 = 15
We can say that the final price of this race was $15, but we don't know how many kilometers were covered. So, we can solve this equation and find the value of p (kilometers). Let's do it?
5p + 3 = 15
5p = 15 - 3
5p = 12
p = 12/5
p = 2.40
Now, we know that the taxi covered 2.4 km charging a fee of r$ 5 per kilometer.