Question 5308
This problem is a problem of translating English into Algebrese.

Gordon figured that when he saved $21 more: saved+21
one-fourth of the money for the camera he wanted: camera_cost/4
he had already saved one-sixth of the amount: saved = camera_cost/6

You have two equations:

{{{system( saved+21 = camera_cost/4,
           saved = camera_cost/6 ) }}}

{{{
 system( saved - camera_cost/4 = -21,
         saved - camera_cost/6 = 0 ) 
}}}

{{{
 system( 4*saved - camera_cost = -84,
         6*saved - camera_cost = 0 ) 
}}}

*[invoke linear "saved", "camera_cost", 4, -1, -84, 6, -1, 0]