24: The derivative

f ′(x) = cos(x) − 2

is always negative, that is, it has no roots. Therefore f itself can have at most one root. On the other hand, since it goes to minus infinity at infinity and it goes to infinity at minus infinity, as a continuous function it must have at least one root (see Intermediate value theorem).

Thus there is exactly one root. Now use the Intermediate value theorem again to identify its position, try substituting nice integer values into f and wait for a sign change between two successive ones.

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