Answer:
Robert used the distributive property.
Explanation:
3(x + 2) = 21 <------- is the original equation.
3x + 6 = 21 <------- is Robert's first step.
Distributive property is a property that states that multiplying the constant by the numerical values/variables in the parentheses will equal the same as multiplying the consonant by the values added together.
Robert multiplied 3 times "x" and "2." This is the distributive property.
I hope this helps! :)