What's the slope of the line ? and explain

You take the difference between any two y and divide by the difference in the corresponding x. The number should be the same always for a straight line. That's your slope. A fancy way of writing it is dy/dx

so: (10-2)/(3-1) = 8/2 = 4 (using the first two points)
or: (26-2)/(7-1) = 24/6 = 4 (using the first and the last)

try it out for other combinations. The answer is 4 every time.