Finding Cosine in an Obtuse Triangle

Consider the triangle ABC, where angle A is known to be obtuse and the lengths of sides BC, CA, and AB are given as follows: $$ BC = a, \; CA = b, \; AB = c $$.

Using the Law of Cosines, we have the relation: $$ c^2 = a^2 + b^2 - 2ab \cdot \cos(A) $$.

If $$ c = 10 $$, $$ a = 7 $$, and $$ b = 4 $$, what is the value of $$ \cos(A) $$?