Question

I have an answer but could you explain better?
The displacement in meter of an object moving in a line given by S-1+2t+1/4t^2 in seconds
a) find the avg velocity over time period.
[1,3]
Answer: interval [1,1+h]
V avg =s(1+h)-s(1)/(1+h)-1=[1+2(1+h)+(1+h)^2/4-13/4]/h=10h+h^2/4h=10+h/4

why its 13//4???