In order to calculate the regression R2 you need the TSS and either the SSR or the ESS. The TSS is fairly straightforward to calculate, being just the variation of Y. However, if you had to calculate the SSR or ESS by hand, you would need all fitted values from the regression function and their deviations from the sample mean, or the residuals. Can you think of a quicker way to calculate the ESS simply using terms you have already used to calculate the slope coefficient? (Tip: use the estimated sploe and the sum of squared deviations of X to express ESS.)