Lets say there's a Transformation function T which takes a 2D vector and returns a 2D vector.
T([p, q]) = [2p + q, p - q]
Lets take a vector [1, 2] and apply the transformation function T on it.
T([1, 2]) = [2(1) + 2, 1 - 2]
T([1, 2]) = [4, -1]
Now lets take another vector [3, 4] and apply the transformation function T on it.
T([3, 4]) = [2(3) + 4, 3 - 4]
T([3, 4]) = [10, -1]
Now lets take a vector [1, 2] and another vector [3, 4] and add them.
T([1, 2] + [3, 4]) = T([4, 6])
T([4, 6]) = [2(4) + 6, 4 - 6]
T([4, 6]) = [14, -2]
Now lets apply the transformation function T on [1, 2] and [3, 4] separately and add them.
T([1, 2]) + T([3, 4]) = [4, -1] + [10, -1]
T([1, 2]) + T([3, 4]) = [14, -2]
The results are same. Hence the function T is a linear transformation.
This function is a linear transformation because it satisfies the properties of linearity.