Suppose you are given the following relationship between a country's sulphur dioxide emissions and its income per capita:

SO₂= 0.083³- 2.2²+ 13.5 + C

whereSO₂represents sulphur dioxide,is income per capita (in thousands of US dollars).C is a positive parametric constant.The level of income per capita where there is a (local)minimumconcentration of sulphur dioxide (to the nearest two decimal places) is: