If f(x) = −2 + 8, what is f(−1.8)? 4 6 10 12

I think that you typed wrongly. That has to be f(x)= -2x+8
If f(x)= -2x+8, we can solve that problem.
We replace x= -1.8 in that expression, that means: f(-1.8)= -2x(-1.8)+8= 3.6+8=11.6

But if the formula of f(x)=-2+8, that means f(x) has a stable value and that value is -2+8=6.
So for all x we always have f(x)=f(-1.8)=6