Max wrote...
Perhaps try, array-entered
(i.e. press CTRL+SHIFT+ENTER):

=SUM(LINEST(A$3:A$6,B$3:B$6)*D2:E2)
....

Better still would be using SUMPRODUCT.

=SUMPRODUCT(LINEST(A$3:A$6,B$3:B$6),D2:E2)

However, if the E2 value were always 1 (for the constant term from the
regression coefficients), it'd make more sense to use TREND or
FORECAST.

=TREND(A$3:A$6,B$3:B$6,D2)

=FORECAST(D2,A$3:A$6,B$3:B$6)

You can drive screws with hammers and pound nails with screwdrivers,
but should you?