Answer
In a vibrating string, a node is a place where the medium does not move while an anti-node has maximum movement (height of peak)
Additionally, this is a closed-closed system (both ends attached) so λ = 2L/m and f=m(v/2L)
Work Step by Step
a. m=4, the string is attached at both ends so you can count the number of antinodes
b. When the frequency is doubles, the wavelength is halved. This will increase the antinodes to 8.
