To find the derivative, you look at each term and multiply the constant in front of the x by the power of x. Also you must lower the power of x by one after this step.
So 5x3 would be differentiated to 15x2. Repeat this for the other terms and you get -4x and +7 (since x1 differentiates to x0 which equals 1). Since -15 is not a term of x it differentiates to 0.
So this equations differentiates to 15x2-4x+7