PAPIs answer in theory is correct, but the SALR and the DALR depend on several variables, not least of which, is the atmosphere stable or unstable, but you don't need to concern yourself with that to any great extent other than looking out of the window.
If you see cumulus or similar cloud, its unstable, if you see strato cu its stable.
All you need to know is, that in the "Standard Atmosphere" ie the assumed day to day weather conditions of any given day at any given time of year, air temperature cools at 2.5 deg C for every 1000ft.
Therefore 21 deg C - 7 deg C = 14, divided by 2.5 deg C = 5.6 (5600ft).