Suppose one of h_1 or h_2 below is a collision resistant hash function

h_1:{0,1}^4n{0,1}^n

h_2:{0,1}^4n{0,1}^n

Prove that for

h:{0,1}^4n{0,1}^2n then h(x)=h_1 (x)||h_2 (x) is a collision resistant hash function