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Replace sin^2(x) by (1-cos^2(x)). You will get
4cos^2(x) - 3sin^2(x) = 4cos^2(x) - 3(1-cos^2(x)) = 7cos^2(x) - 3. ANSWER
Solved, completed and explained.
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To avoid a non-mathematical accent, use a proper Math language:
" express 4cos^2(x) - 3sin^2(x) in terms of cos(x) ".