.
To answer the question, you need to solve this quadratic equation
-0.02x^2 + 0.26x + 7.5 = 8.3,
where left side is your given function and the right side is your given value.
Write it in the standard form and solve using the quadratic formula.
Happy calculations (!)
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On solving quadratic equations using the quadratic formula see the lessons
- Introduction into Quadratic Equations
- PROOF of quadratic formula by completing the square
in this site.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this textbook under the topic "Quadratic equations".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.