1 pencil and 1 pen cost $8 together.

2 pencils and 3 pens cost $21 together.

What the cost of 1 pencil and 1 pen?

I know that the answer is $3 for a pencil and $5 for a pen, but i don't know what equation to use.

Since you posted this to "Pre-Algebra" and haven't even picked any variables, I'll assume that you don't know about variables and other algebraic stuff. So let's use plain arithmetic.

i. You are given the cost of one pencil and one pen. What then will be the cost of two pencils and two pens? (Hint: Multiply.)

ii. You are given the cost of two pencils and three pens, and you just found the cost of two pencils and two pens. What then is the cost of one pen? (Hint: Subtract.)

iii. You are given the cost of one pencil and one pen. You just found the cost of one pen. What then is the cost of one pencil? (Hint: Subtract.)

If you get stuck, please reply showing all of your working in answering the above questions. Thank you!