Question 1189354
{{{(12/2)(A+A+d(n-1))=168}}}


{{{6(2A+nd-d)=168}}}


{{{6(2A+12d-d)=168}}}


{{{highlight_green(6(2A+11d)=168)}}}


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{{{highlight_green(A+2d=7)}}}


The two outlined equations are linear in two unknown variables.   Solve.