[tex]\displaystyle\bf a_{7} = 63 \\ a_{12} = 73 \\ a_{n} = a_{1} + (n - 1)d\\ \\ \left \{ {{a_{7} = a_{1} + 6d} \atop {a_{12} = a_{1} + 11d}} \right. \\ \displaystyle\bf\\\left \{ {{a_{1} + 6d = 63 \: \: | \times ( - 1) } \atop {a_{1} + 11d = 73}} \right. \\ \displaystyle\bf\\ + \left \{ {{ - a_{1} - 6d = - 63 } \atop {a_{1} + 11d = 73}} \right. \\ \\ 11d - 6d = 73 - 63 \\ 5d = 10 \\ d = 10 \div 5 \\ d = 2[/tex]