4x + 9y = 29

x = 5y - 87

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