Answer
$6.75$ dollars
Work Step by Step
$y=\lfloor x \rfloor$ is a greatest integer under $x$. If $x$ itself is integer then $\lfloor x \rfloor=x$.
The given value of $x$ is $7.5$. So, $\lfloor x \rfloor=7$.
The given model:
$$f(x)=0.75\lfloor x \rfloor+1.50$$ $$f(7.5)=0.75\lfloor 7.5 \rfloor+1.50=0.75\cdot 7+1.50=5.25+1.50=6.75$$ The cost is $6.75$ dollars.