Determine the specific gravity of a fish which masses 320g in air and has an apparent mass of 21g in water

Allow me to use units of grams weight, to keep things simple.

Buoyant force in water = 320 - 21 = 299 g
= Volume * (water density)
= 1.000 g/cm^3* V
V = 299 cm^3

Fish density = (Fish weight)/V
= 320/299 = 1.070 g/cm^3

The specific gravity is the fish density divided by the water density, or just 1.070