  We describe the computation of the amplitude A_2 for a kaon to decay into two
pions with isospin I=2. The results presented in the letter Phys.Rev.Lett. 108
(2012) 141601 from an analysis of 63 gluon configurations are updated to 146
configurations giving Re$A_2=1.381(46)_{\textrm{stat}}(258)_{\textrm{syst}}
10^{-8}$ GeV and Im$A_2=-6.54(46)_{\textrm{stat}}(120)_{\textrm{syst}}10^{-13}$
GeV. Re$A_2$ is in good agreement with the experimental result, whereas the
value of Im$A_2$ was hitherto unknown. We are also working towards a direct
computation of the $K\to(\pi\pi)_{I=0}$ amplitude $A_0$ but, within the
standard model, our result for Im$A_2$ can be combined with the experimental
results for Re$A_0$, Re$A_2$ and $\epsilon^\prime/\epsilon$ to give
Im$A_0/$Re$A_0= -1.61(28)\times 10^{-4}$ . Our result for Im\,$A_2$ implies
that the electroweak penguin (EWP) contribution to $\epsilon^\prime/\epsilon$
is Re$(\epsilon^\prime/\epsilon)_{\mathrm{EWP}} = -(6.25 \pm
0.44_{\textrm{stat}} \pm 1.19_{\textrm{syst}}) \times 10^{-4}$.
