Question 357859
ok so i solved this myself it was just a silly order of precedence problem i was having 

when i got
 {{{315 = (21/2) (2a + 60)}}}  i should have 
 {{{315 = 10.5   (2a + 60)}}}  multiply everything in the brackets by 10.5
 {{{315 = 21a + 630}}}
 {{{315-630 = 21a  }}}
 {{{-315 =   21a   }}}
 {{{ -15 = a       }}}

:)