What single gate can be applied to undo the following gate sequences acting on the

qubit state $|$: $?$ If the gate sequences were applied in reverse order how

would your responses change?

$($a$)U=HYZ,$ where $H$ is the Hadamard matrix and $x,Y$ and $Z$ are the Pauli matrices

$($b$)U=xY,$ where $x,Y$ and $Z$ are the Pauli matrices