// Define the spacing unit
$spacing-unit: 24px;

.landing-page {
  
  .landing-section {
    margin-bottom: $spacing-unit*2;
  }
  
  .landing-section h3 {
    margin-bottom: $spacing-unit;
  }
  
  .landing-excerpt {
    font-size: $spacing-unit;
    line-height: $spacing-unit*1.25;
  }
  
  @media only screen and (min-width: $breakpoint-desktop) {

    .landing-section {
      margin-bottom: $spacing-unit*3;
    }
    
    .landing-section h3 {
      font-size: $spacing-unit*2.5;
      line-height: $spacing-unit*2.5;
    }
    
    .landing-excerpt {
      font-size: $spacing-unit*1.5;
      line-height: ($spacing-unit*1.5)*1.25;
    }

  }
  
}

// Define the spacing unit
$spacing-unit: 24px;

.landing-page {
  
  .landing-section {
    margin-bottom: $spacing-unit*2;
  }
  
  .landing-section h3 {
    margin-bottom: $spacing-unit;
  }
  
  .landing-excerpt {
    font-size: $spacing-unit;
    line-height: $spacing-unit*1.25;
  }
  
  @media only screen and (min-width: 1025px) {

    .landing-section {
      margin-bottom: $spacing-unit*3;
    }
    
    .landing-section h3 {
      font-size: $spacing-unit*2.5;
      line-height: $spacing-unit*2.5;
    }
    
    .landing-excerpt {
      font-size: $spacing-unit*1.5;
      line-height: ($spacing-unit*1.5)*1.25;
    }

  }
  
}

Change breakpoint to an actual value
Change breakpoint to an actual value