Concentration in %(w/v) please help how do I solve this?

Question

Five standard solutions of a newly discovered inorganic salt were used to create a calibration curve. The density (g/mL) of each solution was plotted on the y‑axis versus the %(w/v) concentration on the x‑axis. The calibration curve was linear and fitted with the following best‑fit line.

𝑦=0.87𝑥+0.137

A student is asked to analyze a solution of this salt with a density of 3.50 g/mL.

 What is the concentration, in %(w/v), of the unknown salt solution?

Answer 1

y = 0.87x + 0.137 linear regression line

y = density (g/ml)

x = %w/v concentration

unknown density = 3.50 g/ml

solve for x (% w/v)

3.50 = 0.87x + 0.137

0.87x = 3.50 - 0.137

0.87x = 3.363

x = 3.87% w/v