How to execute getactive session function before compare excel function angular 8 ?

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Typescript

angular-cli

Angular8

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 make it please ?
first function i need to execute is
getactivesession

second function i need to execute it
compareexcel
so how to do that please ?

What I have tried:

 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' });
    
  }

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');
       });