The numbers that could fill in the spots are 9 and 3.5.
The equation would look like so:
3x + 2.5 = 12x - 9x + 3.5
That simplifies when combining like terms to:
3x + 2.5 = 3x + 3.5
Because the expression to the right will always have a result of exactly 1 more than the expression on the left, there is no defined solution. 2.5 will never equal 3.5, so the equation has no solution.
The equation is complete to have no solution when the blanks are:
9, and 3.5