The amplitude of a sinusoidal wave is half the distance between the y-coordinates of minimum and maximum points. Or, if you prefer, the distance between the midpoint and the minimum/maximum.
In this case, you can see that when the maximum is reached we have [tex] y=6 [/tex], and similarly for the minimum, [tex] y=-4[/tex].
Half the distance between max and min is thus
[tex] A=\dfrac{6-(-4)}{2} = \dfrac{10}{2} = 5 [/tex]
