X/2 +3y = 6 . . . . . . . . starting point
x/2 = 6 -3y . . . . . . . . subtract terms that don't have x
x = 2(6 -3y) . . . . . . . . multiply by the inverse of the x-coefficient
x = 12 -6y . . . . . . . . . expand if you like
3y = 6 -x/2 . . . . . . . . subtract terms that don't have y
y = (6 -x/2)/3 . . . . . . . multiply by the inverse of the y-coefficient
y = 2 -x/6 . . . . . . . . . expand if you like
Do you want us to answer or simplify?
Simplify:=−18x^7 y^3 + 21x^5 y^4+ 15x^2 y^5
3(k-x)= -3x-9
3k - 3x = -3x - 9 ← equation without parentheses
3k - 3x + 3x = - 9
3k = -9
k = -9/3
k = -3 ← simplest form
Smallest number of cakes that each would have decorated is 18.
