What is the rule for punctuating a dependent clause when it is at the
beginning of a sentence?"

Note that the clauses are separated with a comma when the dependent clause comes first. Some common subordinating conjunctions are: after, as, before, once, since, until, and while.

it would take 95 years for that supernova to be visible on earth, so the year would be about 2113. i hope i did my calculation right.

ek = (m·v2) / 2

v - velocity of system after the collision

m = m1 + m2

ek = (m1 + m2) ·v2/2

ek = (200 + 250) ·2,32/2 = 450·5,29/2 = 1190,25 j