"Four buses were filled with children...each bus holds 58 children" means that you have 4 groups of 58 children in buses. You could model this as 58+58+58+58, but the faster way is 4x58. Either way, you have 232 children traveling in buses to the field trip.
You also have 20 children on the field trip who did not ride the bus, but came in cars. You have to add these children to the children who came in the buses. This is modeled as 232 + 20=252 children in all on the field trip.
To write the entire equation at once, you could use the order of operation rules, which state that multiplication/division should be done before addition/subtraction, or you could specify "first find the students who came on buses" using parentheses. The equation would be: (4 x 58) + 20 =252 children
