[ 3, 4 ] = ( [ (4)^2 - 7(4) + 15 ] - [ (3)^2 - 7(3) + 15 ] ) / [ 4 - 3 ] = 0 m/s
[ 3.5, 4] = ( [ (4)^2 - 7(4) + 15 ] - [ (3.5)^2 - 7(3.5) + 15 ] ) / [ 4 - 3.5 ] = 0.5 m/s
[ 4, 5 ] = ( [ (5)^2 - 7(5) + 15 ] - [ (4.5)^2 - 7(4.5) + 15 ] ) / [ 5 - 4.5 ] = 2.5 m/s
[ 4, 4.5 ] = ( [ (4.5)^2 - 7(4.5) + 15 ] - [ (4)^2 - 7(4) + 15 ] ) / [ 4.5 - 4 ] = 1.5 m/s
To approximate the instantaneous velocity at t = 4......use this interval
[ 4, 4.001 ] = ( [ (4.001)^2 - 7(4.001) + 15 ] - [ (4)^2 - 7(4) + 15 ] ) / [ 4.001 - 4 ] ≈ 1.001 m /s
So....the instantaneous velociy at t = 4 = 1 m/s
[ If you take Calculus....you will see that this is true...!!!!]