Question

X+Y=98 (X-Y=15)

Answer 1

X=15+Y
Put this into the equation on the left.

Answer 2

x=98-y

now you have...

(98-y)-y=15

go for it ...

Answer 3

It's the same, if you follow @Noliec steps or if you follow mine, You'll get the same result (if you solve it right ) ... ;)

Answer 4

Go here: http://www.wolframalpha.com/