Answer based on context:

The 1970s saw a number of new models added to the Buick lineup including the Estate Wagon as its own model in 1970, Buick Centurion in 1971, Buick Apollo in 1973, and Buick Skyhawk in 1975.  1975 also saw the first appearance of the "Buick Park Avenue" nameplate for Buick as a trim/option package on the Electra 225 Limited.  A Buick Century paced the Indianapolis 500 race not once but twice in the mid 1970s.  For 1976, Buick began selling rebadged Isuzu Geminis as Opels to replace the Opel Kadett models it had previously marketed.  The following year, Electra 225 and LeSabre were redesigned and downsized and the Buick brand saw its best model year sales to date with 773,313 vehicles sold.  1978 marked Buicks 75th anniversary and welcomed a redesigned Century as well as a redesigned Regal coupe which was now available with a turbocharged Buick V6 engine engine.  Buick model year sales broke another record in 1978 with 795,316 vehicles sold.  For 1979, the Riviera was redesigned; Riviera S-Type was named Motor Trend Car of the Year.

How many years after the Buick Apollo was added was the Buick Skyhawk added?
2