Firstly we notice that we are dealing with an unentangled pair of qubits.
In vector form the transformation we need is 
$$
\frac{1}{2}\begin{bmatrix}1\\\ 1\\\ 1\\\ 1\\\ \end{bmatrix} 
\rightarrow 
\frac{1}{2}\begin{bmatrix}1\\\ 1\\\ 1\\\ -1\\\ \end{bmatrix}
$$

All that needs to happen to change the input into the goal is that the $|11\rangle$ basis state needs to have its sign flipped.

We remember that the Pauli $Z$ gate flips signs in the single qubit case, and that $CZ$ is the 2-qubit version of this gate. And indeed, the effect of the $CZ$ gate is exactly the transformation we're looking for here.

@[solution]({
"id": "multi_qubit_gates__two_qubit_gate_2_solution_a",
"codePath": "./SolutionA.qs"
})
Alternatively, we can express this gate using the intrinsic gate Z and its controlled variant using the Controlled functor:

@[solution]({
"id": "multi_qubit_gates__two_qubit_gate_2_solution_b",
"codePath": "./SolutionB.qs"
})
