x^2+3x-2 is a factor in f(x) so we can write: f(x)=(x^2+3x-2)(x^2+3x-2-10)=(x^2+3x-2)(x^2+3x-12). When f(x)=-16 we can split -16 into factors we can represent as a*b. If x^2+3x-2=a and x^2+3x-12=b, we may be able to find values of x for a and b. If we complete the square for x we have x^2+3x+9/4=(x+3/2)^2. Set this perfect square to a+2 and b+12 and see what we get: x+3/2=sqrt(a+2) and x+3/2=sqrt(b+12), so a+2=b+12. We know ab=-16 so b=-16/a and a+2=-16/a+12. So a^2+2a=-16+12a or a^2-10a+16=0, which factorises to (a-8)(a-2) and a=8, making b=-2, and a=2, making b=-8. So x+3/2=+sqrt(10) and x+3/2=+sqrt(4). There are 4 values for x: 3/2+sqrt(10)=4.6623, 3/2-sqrt(10)=-1.6623, 3/2+2=7/2, 3/2-2=-1/2.