The range of a projectile is R = (u² sin 2θ)/g.

Step 1. Correct form. $\dfrac{[\text{L}^2\,\text{T}^{-2}]}{[\text{L}\,\text{T}^{-2}]} = [\text{L}]$, a length. It may be right.

Step 2. The student's form. $\dfrac{[\text{L}\,\text{T}^{-1}]}{[\text{L}\,\text{T}^{-2}]} = [\text{T}]$, a time. It is wrong.

Step 3. (b). $R = \dfrac{(15\ \text{m/s})^2 \times \sin 80^\circ}{9.8\ \text{m/s}^2} = 22.6\ \text{m}$.