[tex]2a = 30 \: | \div 2[/tex]
[tex]a = 15[/tex]
[tex]b = 2c[/tex]
[tex]a + b + c = 36[/tex]
[tex]15 + 2c + c = 36[/tex]
[tex]2c + c = 36 - 15[/tex]
[tex]3c = 21 \: | \div 3[/tex]
[tex]c = 7[/tex]
[tex]b = 2c[/tex]
[tex]b = 2 \times 7[/tex]
[tex]b = 14[/tex]