exam Maths Puzzle Numerical Ability

can you show me how to solve f(x)= x^2 +4
If I can see the example to find domain...I think I can explain to my son. I understand what domain is just not how to set up the problem and work it.

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exam Other Question
A certain sum of money is invested at 10%. Twice that amount is invested at 8%. The total amount of interest from both investments is $91.00. How much is invested at 8%. Can you show me how you work it out and solve it? Change the equation -2x + 8y =4 to slope intercept form.
Here are the steps I have preformed....
8y = 4 + 2y
y = 1/2 + 1/4
Is this right?