Problem 1)
The given equations are
y = -x + 4
y = 3x
We would substitute y = 3x into y = -x + 4. It becomes
3x = - x + 4
Adding x to both sides of the equation, we have
3x + x = - x + x + 4
4x = 4
Dividing both sides of the equation by 4, we have
4x/4 = 4/4
x = 1
Substituting x = 1 into y = 3x, we have
y = 3 * 1
y = 3
