The derivative is calculated by multiplying by the degree of the variable then taking taking away 1 from the degree.So to calculate the derivative of x^2 we multiple the variable (which is x) by the degree of the variable (which is 2) then take 1 off the degree of the variable (2-1) to get 1. This means we end up with 2x^1=2x.We can do the same with -6x and so get the derivative is -6x^0=-6 (as x^0=1).Finally the derivative of 20 is 0 as this is the same as writting 20x^0, the first step of calculating the derivative is multiplying by the degree of the variable which in this case is 0. 0 times by any number is 0 so this means the derivative is 0.Putting this all together means that the derivative of x^2-6x+20 is 2x-6.