In a perfectly elastic one-dimensional collision between two bodies of equal mass, the bodies exchange their velocities. Given that the masses are identical ($m_A = m_B$) and the initial velocities are $u_A = 0.5 \text{ m/s}$ and $u_B = -0.3 \text{ m/s}$. After the collision, their final velocities will be $v_A = u_B = -0.3 \text{ m/s}$ and $v_B = u_A = 0.5 \text{ m/s}$. The question asks for the velocities of B and A respectively, so the order is $v_B$ then $v_A$, which is $0.5 \text{ m/s}$ and $-0.3 \text{ m/s}$.