Extrapolation means extending your straight line beyond the range of the points you actually measured, in order to estimate a value you did not record. A common example is extending a cooling graph to predict a temperature at a later time.
To do it, continue the best-fit line in the same straight direction past the last data point, usually drawn as a dashed line so it is clearly distinguished from the measured region. Then read the required value from the extended line using the scale.
Extrapolation carries an important caution: it assumes the same straight-line relationship continues to hold beyond the data, which is not always true in reality. The further you extend, the less certain the estimate becomes.
Used carefully, extrapolation is a valid technique, but you should recognise its assumption and not treat a far-off extrapolated value as if it were measured.
What examiners award
Marks are awarded for extending the best-fit line in the same direction, usually dashed, and reading the estimated value correctly from the scale.
Common mark losses
In short
Extrapolation extends the best-fit line beyond the measured data to estimate a value. Use a dashed extension and treat the estimate with caution.
A routine that works
Practise the mechanics until they are automatic: label both axes with units, choose a scale that fills more than half the grid, plot points with a sharp pencil, draw a single best-fit line, and take the gradient from a large triangle with its coordinates and unit shown. One clean example beats re-reading the theory.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)