The tension in a long string is $25\mathrm{.}0$ N$.$ You oscillate one end of the string up and down with frequency $45\mathrm{.}0$Hz$.$ When this end is at its maximum upward displacement, the nearest point that is at its maximum negative displacement is $0\mathrm{.}400$ m down the string. What is the speed of waves on the string?

What is the linear mass density of the string? $$