Question 95395
First we need to make  "X" the number of scientific calculators 
Second  X+15 will equal the total of graphing calculators.

the scientific calculator is $18 so you want to multiply that by X

the graphing calculator is $87 you will want to multiply 87 times  x+15

Set up the equation like this:

equation set up
$18x + $87(x+15) = $4980         
   

first do the multiplication $87(x+15)
$18x + ($87x + $1305)  = $4980       
 

now add the to variables $18x and $87x
$105x   + $1305  = $4980  
      

now you want to get the variable on one side by itself so you need to subtract $1305 from both sides
$105x   - $1305  = $4980 - $1305


Now you need to divide both sides by the variable  105x
105x    = $3675        

                                    
105x = $3675
____   _____

105  =  105
  x = 35   
      
35 is the total number of scientific calculators
                           

Now you need to add the total of scientific calculators to 15

35 + 15 = 50      50 is the total of graphing calculators