3(x+5)=3x(x+15) what math property is this

Answer 1

None that I know of.

However, the distributive property of multiplication over addition says that

3(x+5) = 3*(x+5) = 3*x + 3*5 = 3x + 15

Are you mixing up x for variables and x for multiply? That's why it's common to use * for multiply