Let x represent pens. Let y represent highlighters.
x + y = 8
y = 8 - x
1.09y + .69x = 7.52
Since we know y = 8 - x, we can replace the y in the equation.
1.09(8 - x) + .69x = 7.52
8.72 - 1.09x + .69x = 7.52
8.72 - .4x = 7.52
-.4x = -1.2
x = 3
You bought 3 pens.
hope this helps!