If we have 2 more blue pens than black pens, our blue pens can be rewritten as blue = 2 + black. Now we can set up an equation. Originally this equation would involve both blue and black, but since we only have 1 equation to set up, we can only have 1 unknown. That's why we base the number of blue pens on the number of black pens and do a substitution. So instead of blue + black = 94, we have (black + 2) + black = 94. That simplifies to 2 black + 2 = 94, and 2 black = 92. Now if we divide by 2, we get that the number of black pens is 46. If we have 2 more blue than black, the number of blue pens we have is 48. 46 + 48 = 94, so there you go!