What is the number of arbitrary constants in the particular solution of a differential equation of order 3?

What is the number of arbitrary constants in the general solution of a differential equation of order 2?

Verify that y=A cos x-B sin x is a solution of the differential equation (d^(2)y)/(dx^(2))+y=0

Mention the order and the degree of the differential equation y_3+ky_2 = 0

Mention the order and the degree of the differential equation y_2=root3(y+y_1^2)

If A={0,1,3} , what is the number of relations on A ?

What is the number of all possible matrices of order 3xx3 with entry 0 or 1?

What happens when Iron nails are placed in copper sulphate solution? Write word equations of the reactions involved.