We call C a function of time t hence we haveC(t) = 60+50t
You want to find C(0), so you put 0 instead of t into the function and getC(t) = 60+50t => C(0) = 60+50*(0) = 60$
What you've done is calculating for which value of t in the function C(t) that gives us the value 0, or in mathematics C(t) = 0.
On the other questions for example, if the plumper took 2 hours, we have C(2)C(t) = 60+50t => C(2) = 60+50*2 = 160$
And on Q4 we want to solve C(t) = 210C(t) = 210 <=> 60+50t = 210 <=> t = 3 hours