X (3 - 5) + (4b) =

x = 8 b = 3

x (3-5) + (4b)

= x (-2) + (4b), since x=8 b=3

= - 16 + 12

= - 4

What I did there is substitution

Replacing x with 8, and b with 3

So - 16 is the product of (-2) and 8

And 12 is the product of 4 and 3

FYI: - 2 is the result of 3 - 5