How o calculate weights in a linear equation?

Context: I have historical data of 4 variable and I want to create an output using these variables. I don't have historical data of the "output".

The idea is to calculate output (y) using a linear combination of the 4 variables (x1, x2, x3, x4):

y = (a1 * x1 ) + (a2 * x2 ) + (a3 * x3 ) + (a4 * x4 )

My question is how to calculate those weights (a1, a2, a3, a4)?

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