Numerical analysis — composite trapezoidal error is $O(h^2)$

Let $f(x)$ take the values $y_0,\dots,y_n$ at $x_0=a, x_1=x_0+h,\dots,x_n=x_0+nh=b$ (the booklet prints $x_1=x_0+n$). Then the error in the trapezoidal rule for integration is of order: