Answer
Al's mass is $96kg$
Work Step by Step
At the start, Al and Jo have a combined mass $M=168kg$, and their velocity $V=0$. After they release their arms, we take Jo's direction to be positive, so $v_J=+1.2m/s$, while $v_A=-0.9m/s$
According to momentum conservation, $$m_Jv_J+m_Av_A=MV=0$$ $$1.2m_J-0.9m_A=0\ (1)$$
We also know that $m_J+m_A=M=168kg\ (2)$
Solving (1) and (2), we find that Al's mass $m_A=96kg$
