a wave travelling along a string is described by y(x,t)=0.005 sin(80.0x- 3.0 t. calculate the displacement y of the wave at a distance x= 30 cm and t=20 s?

This is a simple substitution using the values of x and t that you've been given into the formula - as long as the units for x isupposed to be cm and t is seconds. Do the calculation and post back with what you get.