So you can only simplify like terms this means that number terms like 5 can’t be added or subtracted from 5x and the x is a separate term this is the same for x^2 and so on as these are different terms and can’t be added or subtracted to other terms, next the signs in between the brackets may change whether you add or subtract so be careful with those a negative and a positive make a negative, two positives make a positive, two negatives make a positive, always check the signs when answering so
No20) (2a+9)-(4a-9) would be 2a+9-4a+9 as the minus changes the signs this then equals -2a+18
Now you can do the harder ones
19) (5a^2+7a-1)+(3a^3-4a+6) =
3a^2+5a^2+3a+5
21) (3x^3+3x^2-4x+5)-(x^3-2x^2+x-4) = 2x^3+5x^2-5x+9
22) (2x^3-5x+1)+(4-6x+x^2) = 2x^3+x^2-11x+5
