Simplify r^4st^7 / 5r^5st^3 simplify 7x^3y^9z^4 /28x^8y^2 simplify 3a^3b * 2a^2b^4 / a^10b^3c^2 explain if you can

Cancel out common factors (subtract exponents of common bases) 
1) r^4st^7 / 5r^5st^3 = st^4 / 5r
2) 7x^3y^9z^4 /28x^8y^2 = y^7z^4 /4x^5
The last one you need to multiply the factors first (add exponents of common factors)
3) 3a^3b * 2a^2b^4 / a^10b^3c^2 = 6a^6b^5/a^10b^3c^2 = 6b^2/a^4c^2

Hope this helps, let me know if you need more explanation!