suggest method to solve
Are you SURE this goes in "Algebra / Pre-Algebra"?
Numerical methods.
Fixed Point Iteration.
Secant Method
Midpoint Method
Newton's Method
and on and on and on...
Bond problems often lead to this difficulty. It cannot be solved directly. Often, it is useful to start your iteration process at the coupon rate. The coupon rate is not the correct answer, but it is usually close enough that you will find the correct answer.