Question

if 9,x,x49 are in proportion find the value of x

Answer 1

x=21
9/x=x/49
x2=441
x=21

Answer 2

If 9,x,x,49 are in proportion then
$x \times x = 9 \times 49 \\ x {}^{2} = 441 \\ x = 21$
Hope it helps you