-2.38 is the answer because 7x-3.4=x/10=-2.38
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Solve the trigonometric equation:

Make a substitution:

and the equation becomes

Rewrite conveniently
– t as
+ t – 2t, and then factor the left-hand side by grouping:

Factor out
2t + 1:

Substitute back for
t = cos x:

Therefore,

where
k is an integer.
Solution set:

I hope this helps. =)
The answer for the question is 4