Polynomials are <em>closed</em> under addition and subtraction, which is fancy mathspeak that means that whenever we add or subtract two polynomials, we <em>always</em> get another polynomial. In that way, they behave a lot like the numbers you're probably already familiar with. Adding and subtracting:
Two integers gives you another integer
Two rational numbers gives you another rational number
belt is 0.86 less than jacket b=j-0.86 add 0.86 to both sides b+0.86=j sub b+b+0.86=82.78 2b+0.86=82.78 minus 0.86 from both sides 2b=81.92 divid both sides by 2 b=40.96