the lenght of a rectangle is fixed at 19 cm. What width will make perimeter greater than 70 cm?

The perimeter is two lengths + two widths, and that has to be more than 70.

2L + 2W > 70

We know that the length is 19, so 2L = 38.

2W + 38 > 70

2W > 70 - 38

Now pick a width that makes that true. There are lots of them to choose from: a million will do. :-)