The longevity of a certain metal construction is determined by the following formula:
$$l = {(7.5 – x)}^{4} + 8.97^{c}$$, where l is the longevity of the construction, in years, x is the density of the underlying material, in $$\frac{g}{cm^{3}}$$, and c is a positive constant equal to 1.05 for this type of metal constructions. For what value of density, x, expressed in $$\frac{g}{cm^{3}}$$, will the metal construction have minimal longevity?