( SOMEONE ANSWER PLEASE ) [ I WILL GIVE BRAINLIEST ] to the person who can explain as to how to do an equation like this. I need an explanation because I want to understand the concept. How do you do an equation like this? ( LOOK AT IMAGE PLEASE )

The u-drive rent-a-truck company plans to spend $13 million on 320 new vehicles. each commercial van will cost $25 comma 000, each small truck $50 comma 000, and each large truck  $80 comma 000. past experience shows that they need twice as many vans as small trucks. how many of each type of vehicle can they buy?

What is the value of x when x + 7 = 21?

Which of the following is the result of expanding the series