y = 1/2x - 7/2
so replace y with 1/2x - 7/2 in ur other equation
2x - 3y = 13
2x - 3(1/2x - 7/2) = 13....distribute through the parenthesis
2x - 3/2x + 21/2 = 13 ...multiply entire equation by 2 to get rid of fractions
4x - 3x + 21 = 26
4x - 3x = 26 - 21...combine like terms
x = 5
2x - 3y = 13
2(5) - 3y = 13
10 - 3y = 13
-3y = 13 - 10
-3y = 3
y = -3/3
y = -1
solution is (5,-1)