Verified answer

5×3^x-4×3^x-1⩾99

5*3*3^(x-1) - 4*3^(x-1) ≥ 99

11*3^(x - 1) ≥ 99

3^(x - 1) ≥ 9

3^(x - 1) ≥ 3^2

x - 1 ≥ 2

x ≥ 3

x ∈ [3, +∞)