Let x be large and y be medium, we can write the two totals as
6x+2y=86
4x+3y=72.75
Taking the first equation we can rewrite it to find an expression for x.
6x+2y=86 => 6x=86-2y < Divide this by 6 and we have x=14.333-0,333y.
Now that we have an expression for x in terms of y, we can subsitute in the second equation as follows
4x+3y=72.75
4(14.33-0.33y)+3y=72.75
57.33-1.33y+3y=72.75
3y-1.33y=72.75-57.33
1.67y=15.42 => y=15.42/1.67
y=9.23
x=14.33-0.33*9.23
x=11.26
Note that I've rounded up to two decimals which creates a slight deviation.