A substance has density ρ, mass m, and volume V. If the volume is tripled, what is the new mass?

Since the density of the substance is ρ (rho),

==> every cm³ of this substance has ρ grams of mass.

Then

==> V cm³ of it has ρV grams of mass.  That's ' m '.

and

==> 3V cm³ of it has 3ρV grams of mass.  That's ' 3m '.