Assume that the random variable X follows a uniform distribution given by the expression 1/20 for -10<=x<=10, else 0. find pdf of Y Y=2X^4

syms X Y Xk k
X=-10+(20)*rand(1,20);
g=2.*X.^4;
g1= diff(g);
k = 1:1:2;
Y=0:1:20;
Xk=(-1).^k.*(Y./2).^(1./4);
h1= subs(g,X,Xk)./abs(subs(g1,X,Xk));
h1s = symsum(h1,1,2);
plot (Y,h1s)

i am getting this error

 ??? Error using ==> times
 Matrix dimensions must agree.