To find the height of the ball after 7.5 seconds using the function \( h(s) = -s^2 + 10s + 3 \), we need to substitute \( s = 7.5 \) into the function.
Calculate \( h(7.5) \):
\[ h(7.5) = -(7.5)^2 + 10(7.5) + 3 \]
First, calculate \( (7.5)^2 \):
\[ (7.5)^2 = 56.25 \]
Now substitute this value back into the equation:
\[ h(7.5) = -56.25 + 10 \times 7.5 + 3 \]
Calculate \( 10 \times 7.5 \):
\[ 10 \times 7.5 = 75 \]
Now substitute all values into the equation:
\[ h(7.5) = -56.25 + 75 + 3 \]
Combine the terms:
\[ h(7.5) = -56.25 + 75 + 3 = -56.25 + 78 = 21.75 \]
Therefore, the height of the ball after 7.5 seconds is:
\[ \boxed{21.75} \]