Answer :
So,
we have two equations here in the first part,
[tex]\begin{gathered} Y=3A-1 \\ Y+A=7 \end{gathered}[/tex]Let us use the elimination method here,
Subtracting equation second from first we get,
[tex]\begin{gathered} Y-Y-A=3A-1-7 \\ 0-A=3A-8 \\ 4A=8 \\ A=\frac{8}{4} \\ A=2 \\ \text{putting it in second equation we get,} \\ Y+A=7 \\ Y+2=7 \\ Y=7-2 \\ Y=5 \end{gathered}[/tex]Hence, we get the required value of weights.