[tex]2x + 7y = 3 \: \: \: | \times 4\\8 x + (2c + 1)y = 12 \\ \\ 8x + 28y = 12 \\ 8x + (2c + 1)y = 12 \\ \\ 2c + 1 = 28 \\ 2c = 28 - 1 \\ 2c = 27 \\ c = 27 \div 2 \\ c =1 3.5[/tex]