5.815 and 6.021 as the sum of two mixed numbers

Remember that 1/1=1 and 2/1=2
also remember tht 1.2=1+0.2 so
5.815+6.021
take 5.815
count down how many places the 5 goes
it goes to the thousanths place so
5.815=5815/1000=5 and 815/1000

take 6.021
count how many places the 1 goes
goes to thousanths so
6021/1000=6 and 21/1000
add
5 and 815/1000+6 and 21/1000=(5+6)+(815/1000+21/1000)=11+826/1000 or
11 and 826/1000

(9x⁴ - x²y² - 5y⁴) - (3x⁴ + 2x²y² - 7y⁴)
9x⁴ - x²y² - 5y⁴ - 3x⁴ - 2x²y² + 7y⁴
6x⁴ - 3x²y² + 2y⁴