exam Maths Puzzle Numerical Ability

graph
x-4y=8

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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. How much is invested at 8%. I need help with the following question. The problem needs to be solved using subsitution. The answer that I got does not agree with the answer in the book.

y=x^2
x^2+y^2=1