Any one iam as stuck on this one as well

What is the equation of the graph below? y=sec(x)-4

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!

∆red ≅ ∆ by rule will give brainiest if possible