а)

[tex]x + y = 7 \\ x = 7 - y[/tex]

б)

[tex]2x + y = 502 \\ 2x = 502 - y \: \: | \div 2 \\ x = 251 - 0.5y[/tex]

в)

[tex]x + 4y = 8 \\ x = 8 - 4y[/tex]

г)

[tex]2x + 3y = 9 \\ 2x = 9 - 3y \: \: | \div 2 \\ x = 4.5 - 1.5y[/tex]

д)

[tex]x - y = 5 \\ x = 5 + y[/tex]

е)

[tex]2x + 7y = 18 \\ 2x = 18 - 7y \: \: | \div 2 \\ x = 9 - 3.5y[/tex]