To solve by substitution:
1. Solve a in the second equation:
[tex]a=21+9b[/tex]2. Substitute the a in the first equation by the value you get in the previous step:
[tex]3(21+9b)+4b=1[/tex]3. Solve b:
[tex]\begin{gathered} 63+27b+4b=1 \\ 63+31b=1 \\ 31b=1-63 \\ 31b=-62 \\ b=\frac{-62}{31} \\ \\ b=-2 \end{gathered}[/tex]4. Use the value of b to solve a:
[tex]\begin{gathered} a=21+9b \\ a=21+9(-2) \\ a=21-18 \\ a=3 \end{gathered}[/tex]Then, the solution for the given system is:a=3b=-2