Answer: The property illustrated by the expression 5x*(3x+4y) is the distributive property, specifically the distributive property of multiplication over addition. This property allows you to distribute a factor to each term inside parentheses. Mathematically, it is represented as:
a * (b + c) = a * b + a * c
In the given expression, 5x is being multiplied by the sum (3x+4y), and applying the distributive property results in:
5x * (3x + 4y) = 5x * 3x + 5x * 4y = 15x^2 + 20xy