Let x= side 1= 19
Let y= side 2=unknown
Let z= side 3=unknown
Since it is an isosceles triangle, two sides are equal, which happen to be y and z
Hence y=z
Perimeter of a triangle = Sum of all three sides
= x+y+z
51=19+2y [since we know x=19 and y=z, we know the perimeter is 51]
Solve now
32=2y
16=y
Since y=z
z= 16
Hence the unknown side's length is 16m