There are several ways you can find the square (x+4)².
Use your knowledge that (x+a)² = x²+2ax+a².
Use FOIL* to compute (x+4)(x+4) = x² +4x +4x +4² = x²+8x+16
Use the distributive property: (x+4)(x+4) = x(x+4) +4(x+4) = x² +4x +4x +4² (same as FOIL for two binomials)
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* FOIL is mnemonic for {First, Outer, Inner, Last} referring to pairs of terms in the factors (x +a)(x +b). The x terms are the First terms. The Outer terms are the first x and b. The Inner terms are "a" and the second x. The Last terms are "a" and "b". You are to form the sum of those pairs of terms to find the product of the binomials.
If nthe shop has a mark up rate of 50% and they cost 300 to buy you multiply 300 by 50% convert 50% to decimal so you do .5*300=150 then you add 150 to 300 which means that the shop sells the skis for 450 dollars.