Coefficient of Determination from SSE and SST

In a linear regression model for sales (y, in tens of thousands of dollars) based on advertising expenditure (x, in thousands of dollars), the regression equation is $\hat{y} = 2.5 + 1.8x$. Given a sum of squared errors (SSE) of 45 and a total sum of squares (SST) of 180, what percentage of the variation in sales is explained by the linear relationship with advertising expenditure?