Given that: y = x + 7
y = 4
y = x + 7
y = 4
substitute the value of y into the above question
4 = x + 7
Isolate x
x = 4 - 7
x = - 3
For the second question
y = x + 5 ------ equation 1
y = 3x ----------equation 2
Substitute the value of y = 3x into the equation 1
3x = x + 5
Collect the like terms
3x - x = 5
2x = 5
Divide both sides by 2
2x/2 = 5/2
x = 5/2
substitute the value of x = 5/2 into the equation 2
y = 3x
y = 3 (5/2)
y = 3 x 5 / 2
y = 15/2