Consider the polynomials $$f(x)=1-12x+3x^2-4x^3+5x^4$$ and $$g(x)=3-2x-6x^3+9x^4.$$ .

For f(x) + c*g(x) to have degree 3, the order 4 terms must vanish.  So

5x⁴ + c*9x⁴ = 0

Can you take it from here?

sorry nut I still can not solve it