Question 157356
{{{3X + 3Y + AX + AY}}}
It's a fact, not easy to find, sometimes
that the letters at the END of the alphabet,
U,V,W,X,Y, and Z
are used for variables that can be any
number, depending on other variables.
-----------------------------------------
The letters at the beginning of the 
alphabet are used for CONSTANT variables,
A,B,C,D,E,F, ETC.
a constant varible can have any value, but
once you pick one, it never changes in the
equation in which it appears.
_________________________________________
Imagine that {{{A = 6.15}}} (or any number)
Then you have
{{{3X + 3Y + 6.15X + 6.15Y}}}
{{{9.15X + 9.15Y}}}
But I don't know what {{{A}}} really is, so
{{{3X + 3Y + AX + AY}}}
{{{(3 + A)X + (3 + A)Y}}} answer