How to execute subscribe firstly before another subscribe angular 8?

Question

I work on angular 8 i have two subscribe

i need to execute first subscribe and after finish

execute second subscribe .

but i don't know how to execute subscribe at sequance ?

first function i need to execute is

getActiveSessions

second function i need to execute it

CompareExcel

so how to do that ?

what i have tried

execute it first

this._dataService.getActiveSessions(flagButton).subscribe(resp => {
      if (resp) {
        alert('There are active session working')
      } else {
        console.log('nooo');
      }
    });

execute it second

this._dataService.CompareExcel(this.selectedoptions,this.fileToUpload).subscribe(resp => {
            if (resp) {
              this._dataService.PostUpload(this.selectedoptions,this.fileToUpload)
          .subscribe(blob => {
          saveAs(blob.body, this.fileToUpload.name +'.xlsx');
           });

for more details

service 
    public getActiveSessions(flagButton:number): Observable<boolean> {
        return this.http.post<boolean>(this.url + 'Z2Delivery/getActiveSessions',flagButton);
      }
     public  CompareExcel(selectedoptionsId:Number,file:any):Observable<boolean> 
      {
        const formData: FormData = new FormData();
        formData.append('file', file,file.name);
        formData.append('selectedoptions',selectedoptionsId.toString());
      return  this.http.post<boolean>(this.url + 'Z2Delivery/CompareExcel',formData);
      }
     PostUpload(selectedoptionsId:Number, file:any):Observable<any>
      {
        const formData: FormData = new FormData();
        formData.append('file', file,file.name);
        formData.append('selectedoptions',selectedoptionsId.toString());
       
        return this.http.post(this.url + 'Z2Delivery/Upload' , formData,{observe: 'response',responseType: 'blob' });
        
      }