Answer :
x^2 + y^2 =100
put y = 6
x^2 + 36 = 100
x^2 = 64
x = 8
now, differentiating given equation with respect to 't'
2x.dx/dt + 2y.dy/dt = 0
put x = 8 y = 6 and dy/dt = 4
2.8.dx/dt + 2.6.4 = 0
we get dx/dt = -3
x^2 + y^2 =100
put y = 6
x^2 + 36 = 100
x^2 = 64
x = 8
now, differentiating given equation with respect to 't'
2x.dx/dt + 2y.dy/dt = 0
put x = 8 y = 6 and dy/dt = 4
2.8.dx/dt + 2.6.4 = 0
we get dx/dt = -3