Me on these! i really don't get it and i have to get them done!

Hey there :)

All these shapes are known as a rhombus.
The are formula of a rhombus is 

1st ) The area is given = 48 in² 
       The diagonals are 1) 8 in and 8 in = 16 in , 2) x in and x in = 2x in

Apply the formula to find x 

48 = 16x

Divide by 16 on either side to isolate x

3 = x 

Check:
Area =  
         = 
         = 48 in²

2nd ) Area is given = 48 ft²
          The diagonals are 1) 4 + 4 = 8 ft , 2) x

Apply the formula to find x 
48 = 
48 = 4x

Divide by 4 on either side

12 = x 

Check:
Area = 
         = 
         = 48 ft²

3rd ) Given diagonals are 19 ft and 38 ft
Area = 
         = 
         = 361 ft²

The expression of this would be 3.75x+y= 23