Question 71254
In the question consumption is denoted by  y
and the formula says :    y = -0.2125t^2 + 2.615t + 56.33
so we will calculate y' & y"  to get the value of t to get maxima .
so,   y' = -0.2125*2*t + 2.615
and   y" = -0.2125*2  which is negative this means that only the maxima appears and no minima or point of inflexion will be there.

the location of maxima can be obtained by putting y' = 0
so, we get t = 6.1529 
this means that the time ( 1996 + 0.1529 ) had the largest consumption but as our consideration is for the period 1990 to 1996 so 1996 has the maximum consumption.

http://www.walterzorn.com/grapher/grapher_e.htm  
  this is a graphing site which can be of help as basically this problem was to check your understanding of graphs.

cheers!