There are three parts in common with both expressions. They all have a coefficient, an x, and a y.
1) Start by finding a common factor of the two coefficients, 15 and 20. What is the biggest number that divides them both? Notice that 15 = 5*3 and 20 = 5*4, so that means 5 can be factored out of both.
2) Next find the common factor of the x's. You have x^3 in the first expression and x^4 in the second. x^3 would be the largest expression that divides both so you can factor it out.
3) Find the common factor of the y's. You have y^2 and y^4, making y^2 the largest expression that can be factored out.
That means 5(x^3)(y^2) is the common factor between those two expressions.
