Incógnitas de um lado, números do outro. Lembrando que, ao mudar de lado, a operação muda.
[tex]4 - 2x = x + 31
\\\\
-2x - x = 31-4
\\\\
-3x = 27
\\\\
-x = \frac{27}{3}
\\\\
-x = 9 \ \ \times (-1)
\\\\
\boxed{\boxed{x = -9}}[/tex]
[tex]5 - 2 \cdot (y - 1) = y + 4 \cdot (-2 + 3y)
\\\\
5-2y+2 = y -8 + 12y
\\\\
-2y - y - 12y = -8 - 5 - 2
\\\\
-15y = -15
\\\\
y = \frac{-15}{-15}
\\\\
\boxed{\boxed{y = 1}}[/tex]
[tex]7a - (2a - 1) = 4 + (a - 6)
\\\\
7a - 2a + 1 = 4 + a - 6
\\\\
7a-2a-a = 4-6-1
\\\\
4a = -3
\\\\
\boxed{\boxed{a = -\frac{3}{4}}}[/tex]
