Explanation:
It's the DISTRIBUTIVE PROPERTY:
m(a + b) = ma + mb

Other properties:
Commutative Property:
of addition: a + b = b + a
of multiplication: a · b = b · a
Associative Property:
of addition: (a + b) + c = a + (b + c)
of multiplication: (a · b) · c = a · (b · c)