Question 357859
{{{315 = (21/2 )(2a + (21-1)3)}}} so i need to solve for a
 {{{315 = (21/2) (2a + 60)}}} 
multiply by (2/21) on both sides
{{{315*(2/21)= (21/2)*(2/21)*(2a+60)}}}
{{{315*(2/21)= (2a+60)}}}
{{{30=(2a+60)}}}
{{{30-60=2a}}}
-30=2a
a=-15
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m.ananth@hotmail.ca