The equations are 

                            Equation (1)

                           Equation (2)

Since the coefficients of the b-terms are same, you can eliminate these terms by subtracting the equations. Subtract the equations to eliminate variable b.

Write the equations in column form and subtract the corresponding columns.

Now Substitute –5 for a in either equation to find the value of b.

                            (Equation 1)

                      (Replace a= -5)

                          (Multiply: )

Apply Addtion proparty of equality: If a = b then

          (Add 15 to each side)

              (Commutative proparty:  )

                                         (Additive inverse property: )

                                    (Add: )

The solution is .

The solution is .