Verified answer

That's right, it can't be -2.5 because that would mean the probability density is negative. It can't be negative anywhere, for any value of x.

The total probability has to add up to 1. Integrate that function from 5 to 10. Set it equal to 1. If this is in fact a probability density function.

But if it is, why does it say "density of a chemical solution" (which would be the mass density, not the probability density)?

But if it's mass density, the problem can't be answered without knowing what the total mass is supposed to be, they way you know the total probability has to be 1. Anyway, -2.5 is wrong for mass density also because that would mean the mass is negative.