I am confused by the following and need some help

You want to find the values for the equation ŷi=a+bxi. What is the value of the slope?

time spent on Bookface predictor variable,

time spent working is the response variable. correlation between minutes spent on Bookface and minutes spent working is -0.5. The standard deviation in Bookface time is 4.86, and the standard deviation of time spent working is 3.50.

Would the formula be something like the following

Yi=4.86+3.50-0.5 = 7.86 I am not sure if I am working this out as the answer is wrong I just would like to know where I went wrong for future learning points.