I do not know the behavior of mixture i.e. it behaves like a liquid or gas or both. If it behaves like liquid use the formula below.

Density of C2 at 10C and 9.9MPa x 0.3 + Density of C3 at same conditions x 0.7

If u are unable to get the properties of individual C2 and C3 go to DETHERM properties database or DORTMUND data bank or consult perrys book or Chempak software.