How can i solve these problems? i'm not sure how to substitute the values into the equations?

To get you started:

V(0) is found by replacing every appearance of t in the formula V(t)=12000+120t by 0:

V(0)= 12000+120(0) = 12000.  This represents the initial volume of the water in the reservoir, in millions of cubic feet (before any pollutants have flown in).

P(0) represents the volume of pollutants (in millions of cubic feet) in the reservoir at time t = 0 (at the beginning).

It is P(0) = 350 + 30(0) = 350 million cubic feet.
           P(t)
C(t) = represents the ratio of pollutants to total reservoir volume.  
           V(t) 
                                                                         350
In the beginning, at time t = 0, we get C(0) = (a ratio)
                                                                         12000
  
                                                                     = 0.029

So pollutants make up about 3% of the total volume in the reservoir at time t = 0.