When solving a question like this, with two quantities and their different units (meters and centimetres in this case) you can begin by converting the values to the same units. this makes the calculations easier.
For converting 10m to centimeters, we will multiply 10 by 100 (because 1 m has 100 cm so 10 will have? multiply 10 by 100)
10 x 100 = 1000cm
minus the centimetres she used up.
1000- 210 =790cm is left.