how to solve by substitution
.3x+3y=1.2(100)
"Substitution" implies there are TWO equations in two unknowns.
You solve one of them for one of the unknowns in terms of the other,
e.g., solve for y in terms of x. THEN, you substitute that value into
the other equation where the y is, and you have one equation with
only one unknown, in this case x. You solve for x and substutute that
into the first equation to solve for y.
Having only one equation makes it impossible to solve for either unknown.