We call C a function of timethence 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