A pion lives for 26ns in its rest frame, what will its lifetime be if measured by an observer in a lab.....?

Your question refers to a principle of special relativity known as "time dilation". In brief, it means that moving clocks tick slow. The pion's clock would tick off its 26ns slower than a rest clock, which might read, say, 30ns. To solve this problem reference "time dilation" in a textbook or on the web.