A=[ai,j]2X2 matrix a11=2, a12=1, a21=-1, a22=0 find the U unitary matrix for A and also U*AU matrix must be triangular matrix?

This is linear algebra question i forgot this subject and i don't have any math book about linear algebra. Please solve this question with how do you solved i must learn this question's algorithm. And my english isn't good. please use basic clauses. Thanks now.