Guys how find square cm in trapezium​

In a graph with several intervals o data how does a constant interval appear? what type of scenario produces a constant interval?

Dakota earned $7.50 in interest in account a and $18.75 in interest in account b after 15 months. if the simple interest rate is 4% for account a and 5% for account b, which account has the greater principal? explain. to make it a little easier, there is an image. good luck!

Ineed in figuring out this problem.