At the throwing position, the initial velocity of the projectile is decomposed into horizontal direction (V water) and numerical direction (V vertical direction).
① It is horizontal and understandable.
② It is vertical. Listen, with the vertical direction as the positive direction, there is a formula:
S = V .t + 1/2 a t？
-800 = 300sinα t + 1/2 g t？
Where the vector: the displacement from the slope to the ground is negative; Minute speed (initial speed), positive upward; Acceleration, negative downward.
Do you understand this?
Of course, you can also take the vertical downward direction as the positive direction, then equation (2) is:
800= - 300sinα t + 1/2 (-g) t？ (Mathematically equivalent to shifting terms. )
Remember, as long as it is uniform linear motion, those motion formulas can be used. For example, in the vertical direction of the problem, there is no need to split it into throwing first and then falling freely. Of course, you can separate it.
Be more detailed, and remember to adopt it when you understand it.