[tex] \frac{a + a + 2 + a + 4}{2} = 249[/tex]
[tex] a + a + 2 + a + 4 = 2\times 249[/tex]
[tex]3a + 6 = 498[/tex]
[tex]3(a + 2) = 498 \: | \div 3[/tex]
[tex]a + 2 = 166[/tex]
[tex]a = 166 - 2[/tex]
[tex]a = 164[/tex]
[tex]a + 2 = 164 + 2 = 166[/tex]
[tex]a + 4 = 164 + 4 = 168[/tex]