Question 836376
This is a linear system.  The prices are unknown.

Let j = price of the jam
Let b = price of the butter.


{{{3j+2b=29}}} and {{{6j+3b=54}}}
Solve these simultaneous equations for b and j.  Simplify the "54" equation first.  It becomes {{{2j+b=18}}}.


Solve the "18" equation for b.
{{{b=18-2j}}};
Substitute into ....
{{{highlight_green(3j+2(18-2j)=29)}}}... and continue...