need help with this dont know were to start

question

suppose X1 and X2 are identically distributed independent standard normal random variables

$\displaystyle

\mu =0, \sigma =1

$

write down the joint pdf of (X1, X2). determine the pdf of the linear transformation y1 =2x1-x2, y2=x1-2x2.

determine the correct range of this random variable.

any help appreciated