a 3.5 kg rectangular air mattress is 2.73m long, 1.1m wide, and .124m thick.

what mass can it support in water befor sinking?
answer in units of kg.

so do i do volume times density=mass?
3.5*2.73*1.1*.124=1.303302

No.

The bouyancy is equal to volume*densitywater(g).

Supported Weight=bouyancy-weightmat
mg=volume*density*g-3.5kg
masssupported=volume*densiy -3.5kg

where density is the density of water.

density of water=1.0*10^3

so,
26*3.9*23*(1.0*10^3)-3.5=2332196.5

To determine the mass that the air mattress can support in water before sinking, you can follow the steps you mentioned.

Step 1: Calculate the volume of the air mattress.
Volume is equal to length multiplied by width multiplied by thickness.

Volume = 2.73m * 1.1m * 0.124m = 0.392514 m^3

Step 2: Multiply the volume by the density of water to find the mass that the air mattress can support.
The density of water is approximately 1000 kg/m^3.

Mass = Volume * Density = 0.392514 m^3 * 1000 kg/m^3 = 392.514 kg

Therefore, the air mattress can support a mass of 392.514 kg in water before sinking.