Linear pairs are the angles which formed a straight line
Look at your figure
Which angle pairs formed a straight line
Angle 1 and angle 5 formed a straight line
The sum of them = 180 degrees
Angles 1, 2, and 3 formed a straight line
The sum of them = 180 degrees
Angles 2, 3, and 4 formed a straight line
The sum of them is 180 degrees
Angles 4 and 5 formed a straight line
The sum of them = 180 degrees
So, the linear pairs are
Angle 1 and angle 5
Angle 4 and angle 5