Question

What is five x plus negative three equals fifteen

Answer 1

You write the equation like this:
$5x + (-3) = 15$

But if you want to solve it then you do it like this:
$5x + (-3) = 15$
$5x - 3 = 15$
$5x = 18$
$x = 18/5$

Answer 2

5x+(-3)=15
+ 3. +3
5x=18
/5. /5
x=18/5

x=18/5 or x=3 3/5