If x = -1, you have
2(-1) + 3 cos(-1) + e ⁻¹ ≈ -0.0112136 < 0
and if x = 0, you have
2(0) + 3 cos(0) + e ⁰ = 4 > 0
The function f(x) = 2x + 3 cos(x) + eˣ is continuous over the real numbers, so the intermediate value theorem applies, and it says that there is some -1 < c < 0 such that f(c) = 0.
