Answer
$x^5-15x^4y+90x^3y^2-270x^2y^3+405xy^4-243y^5$
Work Step by Step
Use the formula for binomial expansion, we have:
$(x-3y)^5=(x)^5+5(x)^4(-3y)+10(x)^3(-3y)^2+10(x)^2(-3y)^3+5(x)(-3y)^4+(-3y)^5
=x^5-15x^4y+90x^3y^2-270x^2y^3+405xy^4-243y^5$