To find the height of the ball after 7.5 seconds, we need to substitute \( s = 7.5 \) into the height function \( h(s) = -s^2 + 10s + 3 \).
Let's calculate \( h(7.5) \):
\[ h(7.5) = - (7.5)^2 + 10(7.5) + 3 \]
Calculate \( (7.5)^2 \):
\[ (7.5)^2 = 56.25 \]
Now substitute this back into the equation:
\[ h(7.5) = -56.25 + 10(7.5) + 3 \]
Calculate \( 10(7.5) \):
\[ 10(7.5) = 75 \]
Now substitute this value:
\[ h(7.5) = -56.25 + 75 + 3 \]
Combine the terms:
\[ h(7.5) = -56.25 + 75 + 3 = 75 - 56.25 + 3 = 75 - 53.25 = 21.75 \]
So, the height of the ball after 7.5 seconds is:
\[ \boxed{21.75} \]