The question is asking for the height the a/c will clear the obstacle.
The known facts:
Obstacle 5400' from the screen height position.
Obstacle height = 320'
Climb gradient 7% or 0.07 after take-off. The take off ends at the 50' point from the wording of this question so this means the climb gradient commences 50' above the ground.
Because the gradient is measured from 50' above ground level then you need to measure the top of the obstacle from the same point ie 50' above the surface. This means the obstacles 'height' for gradient purposes is
320'-50' = 270'.
To find a/c clearance you need to find how high the a/c will have climbed above the 50' point when it gets 5400' along track.
Gradient = 'rise over run'
The problem supplies the climb gradient & the 'run' distance so transposing the formula to isolate the 'rise' part gives:
(Gradient x run) = rise
Once you have that 'rise' figure ie climb height above the 50' level, subtract the obstacle's height above this 50' level (270') from it to find the clearance.