How to predict next value using only one variable in linear regression



I am currently learning simple linear regression and I came across to question is that how to predict next value if we know only one variable.

for example I want to know tip for 7th meal ?



This is a time series problem, if you want to use only the outcome of previous instances - you can look at ARIMA models to solve this.

If you can add more features / variables, regression would make more sense.