I would really appreciate it if the steps are detailed and clear. Please do no use wronskian method for this variation of parameters problem .

please help me with the following. I would really appreciate it if the steps are detailed and clear. Please do no use wronskian method for this variation of parameters problem

. use y1v1’+y2v2’=0 and 

y1’v1’+y2’v2’=f(t)/a method ONLY.