Answer: Distributive Property
The term "distribution" is also used in place of "distributive"
This property says
x*(y+z) = x*y + x*z
We multiply the outer term x with each inner term (y and z). Then we add the results.
In this case,
x = 5
y = a
z = 3b
So,
5(a+3b) = 5*a + 5*3b = 5a+15b