The density of the unknown liquid is 0.981 g/mL
Your density triangle tells you that
D = m/V or density = mass/volume
So, D = (15.2 g)/(15.5 mL)
You divide numbers by numbers and units by units.
D = 15.2/15.5 g/mL =0.980 645 161 g/mL (by my calculator)
The answer can have no more than three significant figures, so you round off to three significant figures and get
D = 0.981 g/mL