

The equations are
\
Equation (1)
Equation (2)
Since the coefficients of the m-terms are same, you can eliminate these terms by subtracting the equations. Subtract the equations to eliminate variable m.
\Write the equations in column form and subtract the corresponding columns.
\
Apply division property of equality: If a = b then
(Divide each side by 2)
(Cancel commmen terms) \ \
(Divide:
)
Now Substitute 4 for n in either equation to find the value of m.
\
(Equation 1)
(Replace n = 4)
(Multiply:
)
\
Apply addition proparty of equality: If a = b then 
(Add 20 to each side)
(Additive inverse property:
)
(Add:
)
(Divide each side by 2)
(Cancel commmen terms)
(Divide:
)
The solution is 
The solution is
.