Another math form a+bf

Question

Let f(x)=x-2i and g(x)=4x+5i. Find (fg)(5)

Answer 1

f(x) = x - 2i

g(x) = 4x + 5i

 

(f*g)(x) = (x-2i)*(4x+5i)

 

= 4x² + 5xi - 8xi -10i²              [i² = -1]

 

= 4x² + 10 - 3xi

 

To find (f*g)(3), plug x=3 into (f*g)(x).