y = Ax + A³

Let us find the differential equation representing it so we have to eliminate the constant A

Differentiate with respect to x

Put back value of A in y

Now for degree to exist the differential equation must be a polynomial in some differentials

Here differentials mean

The given differential equation is polynomial in differentials

Degree of differential equation is defined as the highest integer power of highest order derivative in the equation

Highest derivative is and highest power to it is 3

Hence degree is 3