a; b; c; d = first; second; third; fourth shelf
a + b + c + d = 250 cm
b = 2a + 18
c = a - 12
d = a + 4
in terms of a, plug all equations into original
a + (2a + 18) + (a - 12) + (a +4) = 250
5a + 10 = 250
5a = 240
a = 48
b = 2(48) + 18
b = 96 + 18
b = 114 cm/1.14 m