For smart ppl who are good at graphs, inputs, and outputs.

I believe the output is f(6) is -1, the output of f(2) is -4, and f(x)=-4 has the outputs of  2, 4, 8, and 10.

I think.

answer: 8 years

step-by-step explanation:

the explanation is that in 8 years the child will be 20 and in 8 years the father will be 40 and 20 is half of 40.

y - 25 = 2 ( x - 10 ) should be the answer