The slope is 1/2, I explain in the other question
Mom is 30 years old when Serena is 10 because 10 x 3 = 30.
They are 20 years apart. [This is key]
Half (.5) of mom's age will be = to Serena's age.
Therefore, when mom is 40 years old, Serena is 20 years old.
We can see that half of 40 = 20.
Hope this makes sense!
You have to subtract $2.50 and $5.05 then you divide that with 3 and you should get $0.85
X< -3 is the answer to your question.