Answer:
x=-7
Step-by-step explanation:
9x + 16 = 4x - 19
Subtract 4x from each side
9x-4x + 16 = 4x-4x - 19
5x+16 = -19
Subtract 16 from each side
5x+16 -16 = -19-16
5x = -35
Divide each side by 5
5x/5 = -35/5
x = -7
There are 99 palindromic numbers between 10 and 1000: 11, 22, 33, 44, 55, 66, 77, 88, 99, 101, 111, 121, 131, 141, 151, 161, 171, 181, 191, 202, 212, 222, 232, 242, 252, 262, 272, 282, 292, 303, 313, 323, 333, 343, 353, 363, 373, 383, 393, 404, 414, 424, 434, 444, 454, 464, 474, 484, 494, 505, 515, 525, 535, 545, 555, 565, 575, 585, 595, 606, 616, 626, 636, 646, 656, 666, 676, 686, 696, 707, 717, 727, 737, 747, 757, 767, 777, 787, 797, 808, 818, 828, 838, 848, 858, 868, 878, 888, 898, 909, 919, 929, 939, 949, 959, 969, 979, 989, 999.
A rectangle has four sides, two equal to the width and two equal to the length. So, the perimeter is

If we know that the width is 5, and the perimeter is 125, we have

In your case, the unkown is called x, but the concept is the same:

where x is the length of the rectangle.