Question 798950
4x+y=28 (1)
y=3x    (2)

replace/substitute y in (1) with the value in (2)
(1) becomes 4x + (3x) = 28
adding the xs together gives
7x = 28
dividing both sides by 7 gives
{{{ 7x/7 = 28/7 }}}
or x = 4
enter this value in (2)
y=3*4 = 12
Check by putting both values in (1):
4*4+12 = 16+12 = 28