Question 880774
a)
For explaining purposes :
Lets say you only wanted to calculate the path of 3 benches, the result would then be : 
1.5 + 4.5 + 1.5 + 4.5 + 1.5 = 13.5
I could also write this as 
3*(1.5) + 2*(4.5) = 13.5
Turning this into a formula :
x*(1.5) + (x-1)*(4.5) = y

So for 17 benches :
17 * 1.5 + 16 * 4.5 = 97
a) the length would be 97 m

for b)
x*(1.5) + (x-1)*(4.5) = 163.5

Using this website's awesome automatic solver :

*[invoke explain_simplification "x*(1.5) + (x-1)*(4.5) = 163.5" ]