exam Maths Puzzle Numerical Ability

The arithmetic sequence x,2x-20,3x-28,... has D1=-16. What is the formula for the nth term ?
I said tn= -16+12 but was marked wrong :(

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exam Other Question

I need to find out what is the natural logarithem and examples? The endpoints of segment FG have coordinates F(1/2a,-b) and G(a,1/4b). What is the expression for the length of the segment?
I think it is :
a) 1/4sqrt4a^2-25b^2 or
b) 1/4sqrt4a