If f : R → R and g : R → R are given by f(x) = sin x and g(x) = 5x² then find gof(x)

Show that a function f : R → R given by f(x) =ax + b,a, b ∈ R, a ≠ 0 is a bijective.
Is A is square matrix of order 3 and |2A| = k|A|, then find the value of k.