You need to use the distributive property to solve this problem
take each side and distribut the number outide of the parenthesis to both terms inside.
on the left side:
5 times x is 5x
5 times 3 is 15
5x+15
on the right side:
3 times 1 is 3
3 times negative x is -3x
3-3x
so you can rewrite the problem as:
5x+15=3-3x
add 3x to both sides
8x+15=3
subtract 15 from both sides
8x=-12
divide both sides by 8
x=-12/8
you can convert this to a mixed number if you choose
x=1 1/2