Hey there!! 
Let us take the cost of 1 feet redwood as ' x ' 
Let us take the cost of 1 feet pine as ' y ' 
For the first scenario : 
70 ft red wood and 80 ft pine 
1 feet redwood - $x 
70 feet redwood - $70x 
1 feet pine - $y 
80 feet pine - $80y 
Total = $351 
70x + 80y = 351 --------------------------------- ( 1 )  
Second scenario : 
1 feet redwood - $x 
100 feet redwood - $100x 
1 feet pine - $y 
60 feet pine - $60y 
total - $420 
100x + 60y = 420 ----------------------- ( 2 ) 
Now let us get both the equations together : 
70x + 80y = 351 ------------- ( 1 ) 
100x + 60y = 420 ------------- ( 2 ) 
Now let us multiply the  the 1st equation with 3 and the 2nd equation with 4 
3 ( 70x + 80y ) = 3 ( 351 ) 
4 ( 100x + 60y ) = 4 ( 420 ) 
............................
210x + 240y = 1053 
400x +  240y = 1680 
...........................
Now let us subtract the first equation from the second ... 
400x + 240y = 1680 
- ( 210x + 240y ) = - ( 1053 ) 
......................
190x = 627 
dividing by 160 on both sides 
x = 627 / 190 
x = 3.3 
The cost for 1 feet or per foot for the redwood is $3.3 
Now substitute the value of x in any given given equation and find out y 
I will substitute in the 1st equation 
70x + 80y = 351 
70 ( 3.3 ) + 80y = 351 
231 + 80y = 351 
Subtracting by 231 on both sides 
80y = 351 - 231 
80y = 120 
dividing by 80 on both sides 
y = 120 / 80 
y = 1.5 
The cost for 1 feet or per foot for the pine = $1.5 
The answers - $3.3 ( redwood ) ; $1.5 ( pine ) 
Hope my answer helps!