What does the delta symbol represent in the equation

The a992 steel rod bc has a diameter of 50 mm and is used as a strut to support the beam. determine the maximum intensity w of the uniform distributed load that can be applied to the beam without risk of causing the strut to buckle. take f.s. = 2 against bucklin

Ineed the answer to question 1 , i’m desperate

Which two particles are found in the center of an atom