You can do this by having h first and multiplying it with the numbers on the second bracket to be h2-4h. Take the other 4 and multiply it to the numbers in the second bracket to be 4h-16. The numbers by now will be h2-4h+4h-16. The -4h+4h will cancel each other and make you remain with h2-16=0. Take -16 to the other side of the equal sign to make it h2=16. Find the square root of 16 to make the answer 4.
You would need to foil it... But seeing that this particular product is what we call "conjugants" (meaning that everything in both of the parentheses are exactly the same (same variable and number), the only difference is the sign in the middle... One is positive one is negative... This means we can just times the first two terms together and the last two terms together..) because as you see when you foil it... The two middle terms cancel... Leaving just the first term and last term together... Accompanied with a negative in the middle. BUT BECAREFUL.. If you use this rule... It ONLY applys when the original problem has OPPOSITE SIGNS... NOT THE SAME... If they were the same then you MUST foil because the two middle terms won't cancel