Solve the following Cauchy$-$Euler equations.

a$)x^2{y}^{\text{'}\text{'}}-x{y}^{\text{'}}-3y=0$

b$)(x-2{)}^2{y}^{\text{'}\text{'}}-3(x-2)y^{\text{'}}+4y=0$;

c$)x^2{y}^{\text{'}\text{'}\text{'}}=2y^{\text{'}}$

d$)x^3{y}^{\text{'}\text{'}\text{'}}+x{y}^{\text{'}}-y=0$

e$)x^2{y}^{\text{'}\text{'}}-2y=sinlnx$

f$)x^2{y}^{\text{'}\text{'}}-4x{y}^{\text{'}}+6y=0$