Set a variable to represent the number of people that are wearing each color. (It can be any letter but I chose x)
First set the equation
5x+3x=56
You would then add up both values to get 8x
8x=56
Then you would divide the 56 by 8 to get
x=7
Then multiply 7 by 3 (the number for the people with white shirts)
21 people are wearing white shirts
35 people are wearing maroon shirts