We start with f(x)=x^2. A horizontal shift left by 10 means we replace x with (x+10). So the translated function is
g(x)=(x+10)^2.
The requested form is a(x–h)^2+k. Rewrite (x+10)^2 as (x–(−10))^2, so a=1, h=−10, k=0. Thus
g(x)=1(x−(−10))^2+0 = (x+10)^2.
You can also expand if needed: (x+10)^2 = x^2+20x+100.