what I would do here is substitute "5y -87" for x in the first ecquation, since the second equation is already in the form "x=..." so:
4*(5y-87)+9y=29
getting rid of the bracket:
20y-348+9y=29
adding up:
29y=29+348
29y= 377
dividing by 29:
y=13
and now we can calculate x, which is 5*13-87=65-87=-22
so x=-22