There are two variables, therefore we need two equations.
#4x-5y=-7# and #color(red)(y=5x)#
#color(red)(y=5x)# means that #y and 5x# have exactly the same value.
#color(red)(y" and " 5x)# are therefore interchangeable.
#4x-5color(red)(y)=-7# has two variables and therefore cannot be solved with one unique solution. There are infinitely many answers.
Replace #color(red)(y)# by #color(red)(5x)#
This gives an equation with only ONE variable and it can be solved.
# " "4x-5color(red)(y)=-7#
#color(white)(xxxxxxx)darr#
#rarr4x-5color(red)((5x))=-7" "# (now the equation has only x)
#4x -25x = -7#
#-21x = -7" "larr div -21#
#x = 1/3#
Now find the value of #y#, knowing that #y = 5x#
#y = 5(1/3)#
#y = 5/3#
